Which version of blender 2.9 exactly is this as i am using 2.92 and i cant recreate this.
Seems like a bug in a later version so maybe jump back a version and see if it occurs there
Viewport-Rotation keys have a different context and is not a math expression… so the ‘/’ means “the inverse”. To get the same thing you would need to do RZ120.
If you want to use a math expression, do it in the toolbar:
Good luck