I haven't seen this talked about as a bug, but the search function on this site doesn't seem to exist.
I noticed that if I invade a planet with a fleet of atleast 2 ships. (Lets say with 1000 troops each) I get 2000 when I battle the planet. When I win, I get a planet with a population of 2000 and a 1 invasion ship with 1000 troops still on it.
Seems I get a net gain of 1000 population and jump start on my next invasion.
I have no patched my game yet, but will soon. If the problem (bonus) is fixed, I will post results.