But it would sometimes lead to weird stuff like what you just mentioned.
This was a feature in one of their much older games so I'm not sure if it's the case here, the feature was intended in that game for countries to be on their ally's front line helping them in their war and any territory taken would be given to the ally instead. It's possible if you were hanging out in British territory for long enough cut off from your own supply chain then you may have been running on British supplies so it's entirely possible it's a feature in the game to give territory to that country instead of you. In the supply mapmode you can see where supplies come from and there needs to be some kind of link to the capital.
The only other thing I can think of is that your troops may not have been running on your own supplies.
