hey thanks for playing, unfortunately I didn't quite understand by 90 degrees off. Does the player not rotate 360 degrees when you move the mouse in a circle above or around him ?
I've just tried to record the problem with OBS: video
It is 90 degrees for me on my screen, but varying angle on the recording.
I have a 4k display running the game in windowed mode (not full screen). So the mouse coordinate space is not the same as the game's viewport.
I suggest to orient your player according to the angle of the vector pointing from the player's global position to the mouse global position.
See