You're probably right about describing it as a bug.
The problem is that (currently, Crusade v2.3) you need to place all your legions on a single field/tile (= acting as a single army) when invading a planet, or else the defenders will crush your diverted (single) legions one by one.
I always lost my invasions (manually, auto) despite having the same superiority as you.
Then I read in some post that you (again, currently Crusade v2.3) have to - manually - place all legions in one tile so that they would actually act as one.
Since then I always win... 