This is an absolute mystery. I get the same behavior in your project. However, if I copy that marker2D and paste it back under same parent (Player), the pasted Marker2D can be rotated no problem, but the original one cannot be.
If I create a new Marker2D + Area2D + CollisionShape under Player, this new one can also be rotated no problem.
I checked the .tscn file in text editor and couldn’t find any issues. Freakier still, if I add rotation to the original Marker2D in the .tscn file itself on hard disk and reload the scene in godot, the rotation disappears! And if I then save the project and check the .tscn file on disk again, rotation is no longer in the file.
This makes no sense.
I don’t know what to tell you other than you got yourself a possessed Marker2D. Thankfully the demon doesn’t jump to new bodies so just copy-paste that Marker 2D and delete the original and you’re good to go.