I added a lot more to it but I don't know how much is left as of 1.19. 
There was a problem in AI having no way to slow down countries it was constricted by rivalry with or attitudes to, but they can (at least on Hard+ in 1.17-1.18) secretly (or not so secretly if you pay attention) work against what they perceive as the greatest threat. Before you claim bipolar, keep in mind there's a lot of stuff visible in GUI that severely limits what AI can do about a growing threat because they must be "friendly" or whatever.
However, I can't recall if it's intended to happen against allies..
There was a problem in AI having no way to slow down countries it was constricted by rivalry with or attitudes to, but they can (at least on Hard+ in 1.17-1.18) secretly (or not so secretly if you pay attention) work against what they perceive as the greatest threat. Before you claim bipolar, keep in mind there's a lot of stuff visible in GUI that severely limits what AI can do about a growing threat because they must be "friendly" or whatever.However, I can't recall if it's intended to happen against allies..