it's been over 10 days that this bug as been reported.
This is a critical bug: Each time you invade a planet with legions, your number of legions increase...
Sadly, i no longer wish to play a game with this bug!
Note: I suggest that you move the 'STATION GARRISON' with the Xeno combat technique' technology, to give the AI a chance to put legions in defense.
For now i just steamroll one planet after another with just one transport !