And I’ve found the culprit:
The issue was that each tick, the ship could be rotated differently, so the possibility space of the reduced vector changes. Violet velocity shows how previously impossible future velocity becomes possible, so with the optimal ship rotation, I am able to turn the vector fully around.
The sad thing is, that this actually isn’t an issue. It should act exactly like that. Car moving South will move north if only steering will be applied. It just seems counterintuitive - possibly because you could do things like these only in extreme cases, like with a car, where the sideways slowdown is incredibly high, but the forward slowdown is almost non-existent at lower speeds).