It doesn't behave that way for me. The rocket will slow down eventually, though if your velocity is super high it could be pretty futile. It just takes time to cancel out your current momentum with reverse thrust. Boosting in the opposite direction should not immediately amortize the velocity because the rockets thrust is often not powerful enough to immediately do so. If you were dropped off the back off a truck at highway speeds on a bicycle facing backwards to your direction of travel and started peddling it would take some time before you were able to slow down and eventually start moving in the direction of your facing.
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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.