diff --git a/DEVGUIDE.md b/DEVGUIDE.md
index cf88719b..34c2ab00 100644
--- a/DEVGUIDE.md
+++ b/DEVGUIDE.md
@@ -248,6 +248,11 @@ make pytest black flake8 mypy
 
 **Test locations**: `test/action/`, `test/web/`, `test/test_tagbot.py`
 
+> **Warning**: `poetry run pytest` can silently run against a stale copy of
+> tagbot installed in the venv's site-packages instead of the local sources,
+> making test results not reflect your edits. Use `poetry run python -m pytest`
+> (or `make pytest`), which puts the repo root first on `sys.path`.
+
 ### What NOT to Do
 
 - ❌ Print statements (use logger)
diff --git a/tagbot/action/repo.py b/tagbot/action/repo.py
index edbfdc81..81b9bf31 100644
--- a/tagbot/action/repo.py
+++ b/tagbot/action/repo.py
@@ -1749,21 +1749,28 @@ def commit_sha_of_version(self, version: str) -> Optional[str]:
         return self._commit_sha_of_tree(tree)
 
     def branches_of_commit(self, sha: str) -> List[str]:
-        """Return short names of non-default remote branches that contain sha."""
+        """Return short names of non-default remote branches that contain sha.
+
+        Returns an empty list if the default branch contains the commit: a
+        commit reachable from the default branch is a mainline release, even
+        if other branches (e.g. feature branches based on it) also contain it
+        (see issue #574).
+        """
         try:
             output = self._git.command("branch", "-r", "--contains", sha)
             default = f"origin/{self._repo.default_branch}"
             prefix = "origin/"
             prefix_len = len(prefix)
-            return [
-                b.strip()[prefix_len:]
-                for b in output.splitlines()
-                if b.strip() and " -> " not in b and b.strip() != default
+            branches = [
+                b.strip() for b in output.splitlines() if b.strip() and " -> " not in b
             ]
+            if default in branches:
+                return []
+            return [b[prefix_len:] for b in branches]
         except Abort:
             logger.debug("Failed to get branches for commit", exc_info=True)
             return []
 
     def is_backport_commit(self, sha: str) -> bool:
-        """Check if the commit is on a non-default branch."""
+        """Check if the commit is only reachable from non-default branches."""
         return bool(self.branches_of_commit(sha))
diff --git a/test/action/test_repo.py b/test/action/test_repo.py
index 2323fc41..4b84c128 100644
--- a/test/action/test_repo.py
+++ b/test/action/test_repo.py
@@ -1517,9 +1517,10 @@ def test_is_backport_commit():
     r._git.command.return_value = "  origin/main\n"
     assert not r.is_backport_commit("abc123")
 
-    # Commit on default branch and another branch
+    # Commit on default branch and another branch → mainline, not a backport
+    # (e.g. a feature branch based on the release commit; issue #574)
     r._git.command.return_value = "  origin/main\n  origin/release-1.0\n"
-    assert r.is_backport_commit("abc123")
+    assert not r.is_backport_commit("abc123")
 
     # Commit on non-default branch only
     r._git.command.return_value = "  origin/release-1.0\n"
@@ -1557,8 +1558,12 @@ def test_branches_of_commit():
     r._git.command.return_value = "  origin/main\n"
     assert r.branches_of_commit("abc123") == []
 
-    # One non-default branch
+    # Default branch plus other branches → mainline, so empty (issue #574)
     r._git.command.return_value = "  origin/main\n  origin/release-1.0\n"
+    assert r.branches_of_commit("abc123") == []
+
+    # One non-default branch
+    r._git.command.return_value = "  origin/release-1.0\n"
     assert r.branches_of_commit("abc123") == ["release-1.0"]
 
     # Multiple non-default branches, no default