i'll explain what i noticed again: the moment i press boost (spacebar), the acceleration ALWAYS increases at first in the direction of the current velocity. Meaning that if i'm going on a straight line until i reach a high speed, turn the rocket the other way around, and press boost, the rocket will accelerate in the direction of its velocity. The physics behaviour would be to immediately start accelerating in the opposite direction, but here there is a few frames of acceleration (can't debug it for sure, but it looks like it's the very frame where i'm pressing spacebar). The physics behaviour is fine afterwards.
So yeah not a problem of velocity, just acceleration.