Papal guard split, the lord still remains the lord of the castle. He was backed by our faction.
There was a brief period after they split where we didn't know what would happen to the castle, so we thought Chevaliers being close by and our friends, should try to claim it. Once we realized the lord was planning on giving it to us (and us to Chevaliers), Tydeus had already attacked the castle. So yes, for a moment there was confusion over the castle, but the lord was always loyal to our faction, and he remained the lord after Papal Guard split.
Chevaliers "conceding" the castle, wasn't conceding to the hero_party, it was conceding their claim on it, back to the original owner (SaleHope), after we realized he was still on board with our faction. Chevaliers original claim on the castle was the same as hero_parties, they both thought it was up for grabs. When I was informed it was still in our vassal's control, I informed Inglorious, and he took my advice to concede the castle back to the original lord.
Whether Chevaliers were "vassals" or not, does not change the fact that SaleHope was the lord, hero_party attacked and took the castle from him. That was the aggressive act, and not returning it was what caused this war.