I think there is an unintended side effect of Station Garrison. This is repeated like Aid Research and Aid Economy if the last item in the queue. The side effect is that it will drain all your legions without knowing. I think the AI is getting hit by this because you will see some AI planets with huge legion sets on planet.

Aid Research and Aid Economy are not so problematic as Garrison Legion because legions are consumable and scarce. If the AI gets hit by this then they may run out of legions more frequently than it should and make it harder for the AI to muster invasion ships.

Couple of suggestions:

1) It would be nice if repeat last item in queue  for these items was configurable on the govern planet screen. It could be another check box on the right hand side.

2) It would be nice of garrison legions could be shifted to ships and become invasion legions and vice versa.