I was fighting the undead scourge crisis and had the ambition selected. One day after finishing a contract I got a notification that one of the cities was overrun by undead. Once I clicked past the notification, the ambition was gone and the undead scourge appears to be over? I was winning at the time so I can’t imagine I lost, but I didn’t get the achievement for beating it or any kind of notification. The ambition simply disappeared and I stopped being offered undead scourge contracts. I do think my brothers got the “gained faith in your leadership” buff.
Also I have been playing for about 10 days after and have not been prompted to pick a new ambition either.