kristoff said:I had this in vanilla, I had this with MM. Playing Poland and having a lot of info pop ups turned on (so it's not like I've missed anything in the log) I repeatedly observe strange, out of nowhere, drops in my relations with Lithuania. Being a senior partner in PU with it and starting from +200 (no abnormal behave, maybe 1-2 wars as allies waged ect.) suddenly I check and see we have +100, month later it's +3... It happened to me many times. What is causing it? There are no insults, throne claims, wars and alikes between us. I have 0 BB, pretty good reputation. It's even hard to tell when this happens, but usually it's there within first 20-30 years of grand campaign.
One important thing you don't mention as having set as popup: events..
IIRC there's a few PU-related events (both multichoice and no-choice) that may drop your relations by a significant amount.