Not certain if this one has been reported yet or if it´s a bug or feuture, but I have a save game as proof of it´s existance.
In the GC with France, I waited for the Dutch revolt to set the lowcountries aflame, and then declared war on Spain. When the war was concluded I saw one of my allies (Papalstates) besieging in cooperation with the Spaniards a rebel Zeeland. Then when the fortress surrendered it became controlled by the papalstates, While not at war with Spain.
In the GC with France, I waited for the Dutch revolt to set the lowcountries aflame, and then declared war on Spain. When the war was concluded I saw one of my allies (Papalstates) besieging in cooperation with the Spaniards a rebel Zeeland. Then when the fortress surrendered it became controlled by the papalstates, While not at war with Spain.