I don't understand how an ally can take over one of my provinces without a rebellion or being at war with me.

Why is my province getting taken over by an ally in my coalition?
-
-
frisianqueen
Approved the thread. -
did it revolt or did you accidently accept a trade that he made which left him with that province? both of those could happen where you would lose the province to your ally without going to war the first is due usually to you not having good morale or you not having enough units in a province the latter was all you if it happened the best you can do is ask for it back.
-
Im guessing not having good morale. And make sure it was your ally, maybe its just one providence of you enemy left.
-
Go to the notifications. You should find the answer there.
-
If it revolts it goes to another player. Keep troops there till Moral is up.
-
Low morale probably