Something weird is happening when the aliens is down to a single battlestation

I've been kicking the aliens out of the jovian system and destroyed their last orbital hab. Afterwards, I bombarded their mining hab on Callisto, destroying all defenses except one last battlestation. I didn't pay attention to whether it was the nukes or something else that did most of the damage, but the last battlestation is somehow invulnerable despite me attacking with twice as strong a fleet (same type of ships) has anyone encountered this issue?