I'v noticed similar behavior from Kol Battleship which has no planet abilities except bombarding it... no matter how many times i clicked stop, with Hold Position, they continued into the mine field !?!?!
Does this have something to do with the Homing Mines ??? I had roughly 30 scouts near my fleet of capital ships but could not stop them from impaling themselves on the mines...
Mines add serious amounts of lag, need some tweaking.... only so many, an effective mine sweeper, something.... Second thoughts... lets just remove mines and expand on the Star Bases.