Possible ground combat bug

[Q]

When I invaded a planet with my troops (large troops with ground canons) I got in a situation where my troops seems to cause no damage. Although about 100 troops were facing about 10 militia they could not win the battle. I lost some of my troops and got a stalemate. Next turn the groung combat resumed with the same result. I then had a look at the planet cargo and there were still some mines and fighters on it. In tactical combat I then "carefully" bombarded the planet till all units were gone. After that my remaining troops captured the planet in the next turn without any problem.
So it seems that units on a planet cause the attacking troops to be ineffective. Anyone seen anything like this???
I would post the savegame file, but it is a game with a modified racial trait (assimilation) so you would not be able to open it unless you installed the entire modification.