It seems to depend on the game speed or fps speed.
If your fps drops, then your view speed drops too, this means if you have like for a really short time no refreshed frame, then you stuck with your view and the game won't take any input from your mouse for that time.
That's why you need to move your mouse for a longer time as you would if you had high fluent fps.

Sadly high fps does't mean stable latency, it is just a counter in a certain time.
In a absolute bad example it could mean that the game spit out 60 frames in a half second and in the other half 0, wich would still count as a full 60 frames per second.
A movie always got a fixed latency, so a movie looks always smoother also when it got lower fps.

Conclusion:
Stable fluent fps with stable fps latency - stable mouse sensitivity
High fps - high mouse input
Low fps - low mouse input

I hope the developers find a way to still use the input from the mouse, when the fps in game drops. Or like i say, -when the game doesn't refresh the frame timely in a fixed or stable latency.

@BenConnor, that is definitely the case, however it's not the only reason the movement is erratic,
when outside, for example cherno, cranking up the settings to very high and that way getting higher fps in one direction, and lower fps in the other, the mouse sensitivity changes, yes, but indoors the view is behaving even more weird

try going up a stair for example, even if you don't move your mouse nearly enough to turn a corner completely, a stair, on top of a tight corridor, seems to "force" your third person camera around the corner due to the collision detection of the camera being very strict (which it must be, of course), resulting in the view not moving as you are doing with the mouse

... i'll try making a video of this behavior, could take a few days though