[2.4.3 WIUP] Combat between hostile parties in a war doesn't work properly

[Pewt]

Holy War against Amalfi, and meanwhile Apulia is also invading them (Venice allied). Venice walks on to a tile I'm sieging and instead of fighting me they start assisting in the siege. The moment the siege completes, *then* they decide to start fighting me. I walk another army to the province to help, but it doesn't join into the battle, instead waiting for the first battle to be lost. This wasn't a one-time thing: combat/lack thereof between myself and the people hostile (But not at war) with me was generally inconsistent and broken.

This lost me a war which should've been an easy win. Reminds me why I don't normally play ironman...