Yeah, I added a new collapse mechanincs for the human player, based on your comments. Thanks!
Your exploit for intentionally going into crazy amount of negative stability (while having city secession every couple turns but not total collapse), and then liberating/gifting many cities away at once for hundrends of +stability was really creative

Creative, but still had to go. Now collapse will definitely happen if you are very unstable.
Big empires still have more chance for city secession if they are unstable with a reasonable amount of - stability.