Not sure whether to put this as a bug report or into game design; since it could be either depending on intentions.

I call/demand an allied AI to help me in a war, they accept, but then they immediately surrender to the enemy AI. I can keep going back and forth and nothing changes, since the allied AI won't say no to me but also doesn't (assumedly) have the war support to fight against the enemy AI.

Here is a reddit thread discussing the same thing: https://old.reddit.com/r/HumankindTheGame/comments/nzjcu5/allied_ai_immediately_surrendering_after_calling/

It seems glitchy and unintentional for it to happen, which is why I chose to put it into the bug report forum.

I think your suspicions are correct. Maybe there should be a flag that they can't support a war due to an utter lack of support. Otherwise they'll take your offer and then give up reparations.

You could potentially use this to cheese the AI as well, ally with a strong AI who you want to take over, bankrupt them by doing this, then break the alliance and war with them while they're vulnerable? Hope this gets seen by the devs nonetheless.