Losing provinces from crusaders

[Knight Templar]

How my allies took province from me while remaining allied (playing as Egys, Spanish are allies, Almos are enemy):

1. turn: my king's army is situated in Tunisia, Almohad army in Algeria and Spanish crusade (objective province also Algeria) in Morroco. I invade Algeria
2. turn: I notice Spanish crusade also joins me in battle for Algeria. I defeated the Almos (Spanish did more harm than good during battle), and province Algeria goes to me, as I had more army (before and after battle) than Spanish crusaders. Crusade remains in Algeria
3. turn: at the beginning of the turn I realised that I have no longer province Algeria. My king's army, who won the battle, is now situated in Tunisia again, and Spanish' crusade holds Algeria. I didn't notice any messages like "province lost" or "the crusade asks you to give the objective province peacfully", and Spanish remained my allies.

This is the second time this happened to me, do you know why is that?

[Mithrandir]

Maybe the crusade's attack was 'planned' in the same turn, just before you.
Then you'd "help/join the crusade" in battle.

I don't remember it being specificly crusades..

[Arkell]

Then you'd "help/join the crusade" in battle.

Yea, sounds like your are helping Spanish not the other way around. But again..

[Weebeast]

That's weird. The owner of Algeria was supposed to be determined as the battle on second turn ended. Is that something I never knew about? It's new to me.