this by itself wouldn't be a significant problem if I hadn't started the invasion specifically to deny my closest neighbors that same planet (Thalan homeworld) + I needed to finish them off before their battleships got back from wherever I warped them to.
So I'm about one turn away from completing the invasion, a late-arriving ship entered the same hex, joined the fleet, and canceled the siege. At which point the Luxar (my neighbor) jumps in to start their own invasion. And of course the only way to cut that short was to go to war with them, which added a bit of overhead to the whole thing.
Is adding another ship to an invasion fleet supposed to cancel the invasion?