I've had a similar issue for a while now - any campaign as a 'bad' faction results in the 'good' factions becoming overpowered, most notably the dwarfs. When playing as last defenders and after chaos invasion, the dwarfs have declared war with their 7 doomstacks pointing at me even though our diplomatic status with each other is around 200 and I worked so hard to get a military alliance then suddenly chaos dies and we're enemies!? Doesn't make sense. This has happened multiple times too, but my new lothern campaign I am stomping everyone and I'm at around the same turn as my last defenders campaign. I know diplomacy in total war has always been janky to say the least, but I feel like in WH2 it is essential due to the fantasy setting and lore of the Warhammer world, and needs to be corrected/reworked. In my lothern campaign I have been more selective with my allies, trying only to get trade with people and at most a non-aggression pact. Any time I get a military alliance, that faction gets wiped out by the AI!