Hmm, I suspect your mouse is actually on the fritz - this kind of behavior can be indicative of a hardware problem.
It could also be some deception/illusion on the player's side, because I remembered something similar.
To test that I started the game, played around a bit with a slipstream and experienced the following:
If you enter a (sufficient wide) slipstream, your fleet gets sucked into it's stream and just "sails" with it for a certain period of time. And this period of time feels longer if you enter the slipstream with sustained burn on.
In the past I thought, entering a slipstream would break the "hold-mouse-button-steering", so I clicked repeatedly again to regain control. Now after your post Alex, I tried it again, kept the mouse button down, and noticed that I still could steer the fleet that way and it was just the slipstream who "caught me inside" for some time.
A possible source for this wrong impression is, that your fleet inside the slipstream looks like it happily follows a set course straight ahead.
If there would be a visual indication, that the fleet tries to escape the grip of the slipstream but it just can't, that would clarify things here.