I'm playing as Ottomans.
I'm not in any coalition or alliances, but I have vassals.
I'm the target of a coalition of Venice, Poland, Bosnia, Persia, Ethiopia, Oman.
I have fabricated at least 1 claim per each of those counties.
I declare war with conquest war goal.
Even if I occupy every enemy province I have claims on, in the peace treaty I can only demand provinces from the one country I declared war on.
They entered the coalition at different times so the truce prevents them from fighting me all at once.
If I declared war on Oman, then Persia and Venice would come to Oman's aid, but Poland, Bosnia, and Ethiopia cannot help Oman because of their truce with me.
I thought I would demand few provinces from Oman, few from Persia, one from Venice, all occupied by me, but in the treaty screen I can only demand from Oman.
and this is odd, because few decades ago, when the coalition members were Mamelukes, Oman, Qara, Georgia, and I had no vassals but I had allies in Tripoli, I could demand provinces from numerous enemies at once. Didn't matter who I declared war on.
Best Answer
Coalitions are an 'anti-blob' measure Paradox added to slow down expansion. Thus, members of the coalition (outside the target of the attack) are not part of the peace negotiation deals.
If in another war you could demand provinces from different countries, they probably were just allies (or cascade allies), but not there because of being in a coalition.