This sort of thing happens every now and again. A state of war is declared by or on some insignificant territory and everyone just forgets.
Do you know Andorra? It's a tiny country between France and Spain. They joined in with WW1 but never actually did anything and were never invited to the negotiating table afterwards, so they didn't technically peace out with Germany until after WW2. Everybody involved on both sides just kind of forgot that they were involved.