Is this by design? When cultists ask for a city when proposing a truce, and the proposition is accepted, the new city isn't destroyed. It happened once in an online game with a friend (so it might have been a one time bug rather than a consistent one) and we were discussing the potential of such a thing if it was intentional.

Could you upload a save game file where this issue is occurring, so that the devs can have a better look ?

I'm'ma try dis! I'll get back to you.