diff --git a/src/wp-includes/html-api/class-wp-html-tag-processor.php b/src/wp-includes/html-api/class-wp-html-tag-processor.php
index ace3e14bea565..3bc22c88eeda4 100644
--- a/src/wp-includes/html-api/class-wp-html-tag-processor.php
+++ b/src/wp-includes/html-api/class-wp-html-tag-processor.php
@@ -752,7 +752,7 @@ class WP_HTML_Tag_Processor {
 	 *     );
 	 *
 	 * @since 6.2.0
-	 * @var bool[]
+	 * @var array<non-empty-string, self::ADD_CLASS|self::REMOVE_CLASS>
 	 */
 	private $classname_updates = array();
 
@@ -809,7 +809,7 @@ class WP_HTML_Tag_Processor {
 	 *     );
 	 *
 	 * @since 6.2.0
-	 * @var WP_HTML_Text_Replacement[]
+	 * @var array<int|string, WP_HTML_Text_Replacement>
 	 */
 	protected $lexical_updates = array();
 
@@ -2968,13 +2968,45 @@ public function get_attribute_names_with_prefix( $prefix ): ?array {
 
 		$comparable = strtolower( $prefix );
 
+		/*
+		 * For the `class` attribute, ensure that enqueued class changes from
+		 * `add_class` and `remove_class` are flushed into attribute updates.
+		 */
+		$has_class = isset( $this->attributes['class'] );
+		if ( '' === $comparable || str_starts_with( 'class', $comparable ) ) {
+			foreach ( $this->classname_updates as $update ) {
+				if (
+					( $has_class && self::REMOVE_CLASS === $update ) ||
+					( ! $has_class && self::ADD_CLASS === $update )
+				) {
+					$this->class_name_updates_to_attributes_updates();
+					break;
+				}
+			}
+		}
+
+		$additions = array();
+		$removals  = array();
+		foreach ( $this->lexical_updates as $update_name => $update ) {
+			if ( is_int( $update_name ) || 'modifiable text' === $update_name ) {
+				continue;
+			}
+
+			if ( '' === $update->text ) {
+				$removals[ $update_name ] = true;
+			} elseif ( ! isset( $this->attributes[ $update_name ] ) && str_starts_with( $update_name, $comparable ) ) {
+				$additions[] = $update_name;
+			}
+		}
+
 		$matches = array();
 		foreach ( array_keys( $this->attributes ) as $attr_name ) {
-			if ( str_starts_with( $attr_name, $comparable ) ) {
+			if ( str_starts_with( $attr_name, $comparable ) && ! isset( $removals[ $attr_name ] ) ) {
 				$matches[] = $attr_name;
 			}
 		}
-		return $matches;
+
+		return empty( $additions ) ? $matches : array_merge( $additions, $matches );
 	}
 
 	/**
diff --git a/tests/phpunit/tests/html-api/wpHtmlTagProcessor.php b/tests/phpunit/tests/html-api/wpHtmlTagProcessor.php
index 84d90a84190fc..66e01dbdbed3e 100644
--- a/tests/phpunit/tests/html-api/wpHtmlTagProcessor.php
+++ b/tests/phpunit/tests/html-api/wpHtmlTagProcessor.php
@@ -464,6 +464,214 @@ public function test_get_attribute_names_with_prefix_returns_attribute_added_by_
 		);
 	}
 
+	/**
+	 * Ensures that a new attribute added via set_attribute() is reported by
+	 * get_attribute_names_with_prefix() immediately after being added.
+	 *
+	 * @ticket 64567
+	 *
+	 * @covers WP_HTML_Tag_Processor::get_attribute_names_with_prefix
+	 */
+	public function test_get_attribute_names_with_prefix_immediately_reflects_new_attributes() {
+		$processor = new WP_HTML_Tag_Processor( '<div existing>' );
+		$processor->next_tag();
+
+		$this->assertSame(
+			array( 'existing' ),
+			$processor->get_attribute_names_with_prefix( '' ),
+			'Failed to report existing attribute names: check test setup.'
+		);
+
+		$processor->set_attribute( 'new', true );
+
+		$this->assertSame(
+			array( 'new', 'existing' ),
+			$processor->get_attribute_names_with_prefix( '' ),
+			'Failed to report newly-added attribute.'
+		);
+
+		$this->assertSame(
+			array( 'existing' ),
+			$processor->get_attribute_names_with_prefix( 'exist' ),
+			'Should have only reported existing attribute matching given prefix.'
+		);
+	}
+
+	/**
+	 * Ensures that changes enqueued for modifiable text do not get reported as added attributes.
+	 *
+	 * This is a fairly-specific test against an internal implementation detail, but is worth
+	 * adding to catch potential regressions since modifiable text updates share a namespace with
+	 * attribute updates.
+	 *
+	 * @ticket 64567
+	 *
+	 * @covers WP_HTML_Tag_Processor::get_attribute_names_with_prefix
+	 */
+	public function test_get_attribute_names_with_prefix_ignores_immediately_added_modifiable_text() {
+		$processor = new WP_HTML_Tag_Processor( '<title existing></title>' );
+		$processor->next_tag();
+
+		$this->assertSame(
+			array( 'existing' ),
+			$processor->get_attribute_names_with_prefix( '' ),
+			'Failed to report existing attribute names: check test setup.'
+		);
+
+		$processor->set_modifiable_text( 'content!' );
+
+		$this->assertSame(
+			array( 'existing' ),
+			$processor->get_attribute_names_with_prefix( '' ),
+			'Failed to report that the attributes were unchanged.'
+		);
+	}
+
+	/**
+	 * Ensures that an attribute removed via remove_attribute() is no longer reported
+	 * by get_attribute_names_with_prefix() immediately after being removed.
+	 *
+	 * @ticket 64567
+	 *
+	 * @covers WP_HTML_Tag_Processor::get_attribute_names_with_prefix
+	 */
+	public function test_get_attribute_names_with_prefix_immediately_reflects_removed_attributes() {
+		$processor = new WP_HTML_Tag_Processor( '<div existing data-removed>' );
+		$processor->next_tag();
+
+		$this->assertSame(
+			array( 'existing', 'data-removed' ),
+			$processor->get_attribute_names_with_prefix( '' ),
+			'Failed to report all existing attributes: check test setup.'
+		);
+
+		$processor->remove_attribute( 'data-removed' );
+
+		$this->assertSame(
+			array(),
+			$processor->get_attribute_names_with_prefix( 'data-' ),
+			'Expected no custom data attributes after removing the only one.'
+		);
+
+		$this->assertSame(
+			array( 'existing' ),
+			$processor->get_attribute_names_with_prefix( '' ),
+			'Expected to report only the attribute which wasn’t removed.'
+		);
+	}
+
+	/**
+	 * Ensures that when the `class` attribute is newly added via add_class(), that
+	 * it’s reported by get_attribute_names_with_prefix() immediately after being added.
+	 *
+	 * @ticket 64567
+	 *
+	 * @covers WP_HTML_Tag_Processor::get_attribute_names_with_prefix
+	 */
+	public function test_get_attribute_names_with_prefix_immediately_reflects_class_after_adding_classes() {
+		$processor = new WP_HTML_Tag_Processor( '<div existing>' );
+		$processor->next_tag();
+
+		$this->assertSame(
+			array( 'existing' ),
+			$processor->get_attribute_names_with_prefix( '' ),
+			'Expected to only report the existing attribute: check test setup.'
+		);
+
+		$processor->add_class( 'added' );
+
+		$this->assertSame(
+			array( 'class' ),
+			$processor->get_attribute_names_with_prefix( 'class' ),
+			'Failed to report the newly-added `class` attribute.'
+		);
+	}
+
+	/**
+	 * Ensures that when the `class` attribute is emptied via remove_class(), that
+	 * it’s reported by get_attribute_names_with_prefix() immediately after being added.
+	 *
+	 * @ticket 64567
+	 *
+	 * @covers WP_HTML_Tag_Processor::get_attribute_names_with_prefix
+	 */
+	public function test_get_attribute_names_with_prefix_immediately_reflects_class_after_removing_all_classes() {
+		$processor = new WP_HTML_Tag_Processor( '<div class="red green" existing class class=duplicate>' );
+		$processor->next_tag();
+
+		$this->assertSame(
+			array( 'class', 'existing' ),
+			$processor->get_attribute_names_with_prefix( '' ),
+			'Expected to find proper existing attributes: check test setup.'
+		);
+
+		$processor->remove_class( 'red' );
+
+		$this->assertSame(
+			array( 'class', 'existing' ),
+			$processor->get_attribute_names_with_prefix( '' ),
+			'Should have the same attributes after removing one of two classes.'
+		);
+
+		$processor->remove_class( 'green' );
+
+		$this->assertSame(
+			array( 'existing' ),
+			$processor->get_attribute_names_with_prefix( '' ),
+			'Should have removed the `class` attribute after removing all class names.'
+		);
+	}
+
+	/**
+	 * Ensures get_attribute_names_with_prefix() agrees with get_attribute()
+	 * after pending updates, returning each name once with no stale entries.
+	 *
+	 * @ticket 64567
+	 *
+	 * @covers WP_HTML_Tag_Processor::get_attribute_names_with_prefix
+	 */
+	public function test_get_attribute_names_with_prefix_immediately_agrees_with_get_attribute_after_updates() {
+		$processor = new WP_HTML_Tag_Processor( '<div data-keep="1" data-drop="2">Test</div>' );
+		$processor->next_tag();
+
+		$this->assertSame(
+			array( 'data-keep', 'data-drop' ),
+			$processor->get_attribute_names_with_prefix( '' ),
+			'Failed to find expected existing attributes: check test setup.'
+		);
+
+		$this->assertSame(
+			'1',
+			$processor->get_attribute( 'data-keep' ),
+			'Failed to find expected existing `data-keep` attribute value: check test setup.'
+		);
+
+		$this->assertSame(
+			'2',
+			$processor->get_attribute( 'data-drop' ),
+			'Failed to find expected existing `data-drop` attribute value: check test setup.'
+		);
+
+		$processor->set_attribute( 'data-keep', 'updated' );
+		$processor->set_attribute( 'data-add', 'new' );
+		$processor->remove_attribute( 'data-drop' );
+
+		$names = $processor->get_attribute_names_with_prefix( 'data-' );
+
+		$this->assertSame(
+			array( 'data-add', 'data-keep' ),
+			$names,
+			'Failed to report the expected attribute names after removing and adding attributes.'
+		);
+
+		foreach ( $names as $name ) {
+			$this->assertNotNull(
+				$processor->get_attribute( $name ),
+				"get_attribute_names_with_prefix() reported '{$name}' but get_attribute() did not agree."
+			);
+		}
+	}
+
 	/**
 	 * @ticket 56299
 	 *