This is actually TWO bugs.
First, I’ve noticed that occasionally, fighter groups will disappear from enemy fleets when they are encountered again. This seems to happen most when you retreat. This isn’t a major bug, except…
If you ever fight a sector that has NO ships in it (the fighters that were there last time having vanished), then the game will crash (GSB.exe yadda yadda). This makes that sector impossible to fight, and I’m not sure what will happen if the fleet moves to attack…