[Niarg]

I've had this happen when two armies met head on (A attacks B and B attacks A). The enemy moved first and attacked my province. I won that battle but then my troops refused to move on and take their land.

I have a feeling this is to do with the way that dominions resolves where the battles are. It tries to work out where everyone will be at the end of the turn and only then does it resolve battles, if your army is slow to move then it gets pinned down in it's own territory. Unfortunately I don't know what constitutes a "slow" army but I've got a nasty feeling that it has got something to do with the invisible unit IDs.