Vertical sensitivity is the highest when moving from center up and from center down and slowest when moving from looking up or down towards center. So when you’re going from looking up to looking down (180 degree move) the speed changes at the horizon level for no apparent reason. It goes slow then like twice as fast.
The problem is smaller on lower sensitivities but unbearable on maximum sensitivity. I literally cannot play after playing Darktide normally.
Aim acceleration, aim assist are both disabled. But actually they make no difference even if enabled.
So turn your sensitivity to 10 across all the sliders and try looking straight up and down.