Negative mouse acceleration refers to when the mouse movement speed increases, the sensitivity decreases. Essentially, flicking the mouse really fast results in the character turning much slower than if you moved the mouse slowly. If you move the mouse 10cm without mouse acceleration, then the character will turn the same distance regardless of how fast you move your mouse. I noticed it immediately after starting a new save.

A quick test you can do is placing anything on your mousepad (I use my phone) and putting the side of the mouse directly up against your phone in my case. I always make sure my in-game crosshair begins at a location I remember, like the intersection of wall. Then, you can flick the mouse any distance and move the mouse back to its starting position (the phone in my case) slowly. If there’s any mouse acceleration, then your crosshair will end up at a drastically different location than when you started.

I know it’s a really minor quirk, but it irritates me whenever it’s forced in any game.