Slitherine

I attacked Antigonids and won a few battles. So their faction collapsed. Then I attacked Macedonia and their faction collapsed. The Seleucids controlled about a third of the map and after I attacked them...they collapsed. While this was happening, Rome had three civil wars, the last with 5 Usurpers, one of whom still holds Rome. So Rhodus is thriving and the big fish aren't even surviving.I don't recall something like this from the play-testing, so...

The AI can sometimes struggle in games if patches introduce new conditions that weren't in the original play tests. Is there anything new from vanilla that the AI might be having trouble wrapping its head around?

I haven't really played enough against the AI to participate in this fully. It can actually be quite good at keeping ahead of large player armies with small raiding stacks which cause havoc. On the other hand, it's not really great at going for the throat. It would be nice if it could prioritise going straight after high resource targets (capitals, high gold/manpower provinces, etc) if it has a military advantage, or circle the wagons around its most valuable/fortified provinces if it is on the backfoot (a good strategy would have it favour standing in a mountain province near valuable resources, only to strike out at the player if they get close to besieging a highly valuable province placed on, say, plains.)

I'm also not sure if it uses the sea lanes to full effect when transporting troops to a front, or if it can prioritise a front effectively to either concentrate against a strong opponent or ruthlessly crush a weaker ally. Having it automatically move its navy to escort any particularly valuable seaborne armies would be awesome, but I'd need Pocus to weigh in on that.

What might be a larger underlying problem is if it can't keep on top of decadence very well. The large Greek kingdoms in particular are exposed to decadence difficulties, which is fairly historical and is hard for the player to stop as well. Getting the Antigonids into the white can be harder than surviving the initial wars against Seleucia and Egypt.