When I'm in 3rd person, standing still, and no weapons/things equipped in hands, my Y sensitivity suddenly drops to low levels. This happens for both mouse and gamepad.

Mouse Y speed returns to normal when I start moving, equip something in hands, or go to first person, it only happens when I'm idle in 3rd person.

I have removed all mods, reinstalled Skyrim untouched from scratch, and removed all local and cloud config files, and this keeps happening for both vanilla and SE versions of Skyrim. I'm sure other people have encountered this as it seems like a software issue, but no one is responding with anything helpful. I'm beginning to think this is just a "feature" but why isn't there some console variable to change it?

Anyone have any ideas?