I’ve been playing through the Georgian campaigns and have now encountered this bug as well. It makes the freeing Trebizond objective completely obsolete.
Also I assume the Armenian Civilians (teal) should be allied to us instead of enemied as well.
I also have the same issue. Since me and the Brothers are allied with him (and I need him for the achievement, rather than ending him there), he’ll also attack them as well, which can be annoying and may prevent them building a base. On this pic they ignore the enemy and focus me. So in the end you have to fight the Turks + your ally is spamming you with attacks and you can’t/shouldn’t retaliate.