For some reason, this fixes the problem of back() not working. I tested it with multiple menus as well, works fine.

Author: Matees1

src/main/java/me/kodysimpson/simpapi/menu/PlayerMenuUtility.java

@@ -76,6 +76,7 @@ public <T> T getData(Enum identifier, Class<T> classRef){
      * @return Get the previous menu that was opened for the player
      */
     public Menu lastMenu(){
+        this.history.pop(); //Makes back() work for some reason
         return this.history.pop();
     }