It's not a bug. See above video. It's because both the crosshair & reticle move away from the centre of the screen in the direction you turn, but at a variable rate and distance depending on the speed of your mouse movement.
Your perception of sensitivity is formed from a combination of not only the game world's velocity relative to your mouse movement, but also this velocity relative to the crosshair. The eye is naturally drawn to the centre of screen as you turn by the projection of the FOV (the angles of the game world all bend away from the centre of the screen, which is called "pincushion distortion"). When the crosshair decouples from this centre there is a subtle contradictory sensation, which is greater when moving fast and less obvious when moving slowly.
Having the crosshair partially move with the game world's turn-rate makes your sensitivity feel much faster when you move faster and less-faster when you move slowly, hence your perception of sensitivity feeling floaty and imprecise, and like it's too fast overall. It's not actually acceleration as far as the game world movement, but as far as the aiming movement - it actually kind of is. This is not an issue with a timing-based aiming device like a control stick but for a direct movement aim device like a mouse it's not intuitive.
I think this is probably ok for some game types, single player or milsim games - but for an Arcade PvP FPS it is probably a bad design choice. I'm sure people will adapt though.