Steps to reproduce:

• move your mouse without riding anything, this is expected behavior
• ride something
• move your mouse. now horizontal movement seems to lag behind vertical

In the attached video, a cursor is drawn directly with the X and Y components of the camera rotation matrix to demonstrate the issue. The behavior can be observed without it, albeit less noticeable. It is possible to see the issue without the custom cursor if you focus on a corner of the stone platform. It may be easier to notice when moving the mouse fast in a large circular motion, or flicking the mouse diagonally