That might just be a generic controller or mouse configuration issue either through your Steam controller settings, or with how it is set up for your system in general. More likely its tied to your calibration for your system as a whole, either via the systems tools or specific tools tied to the controller/mouse itself.

Another possibility is if you are using a controller, is your controller has developed an expanded deadzone so that its not registering a direction as fully as it should.

Its the same for controller and mnk. 1st person is consistent no problems with looking up and down. When i'm in third person while standing still and not moving, The Y axis is super sluggish. When i move in 3rd person the sensitivity changes and is faster. Iv'e tried changing my ini files but nothing works.