It looks like a few modders managed to discover what's probably causing the issue.
The game constantly checks for requirements to allow things like certain interactions, one of those requirements are for sims to be able to do mean interactions, this is done through tests. When they added the loyal trait they added a test set which unfortunately included a test for Snowy Escape.
If you don't have Snowy Escape, the test will be empty, allowing all sims to be mean autonomously. This is most likely what has caused all sims to be mean all the time.
Reading about this bug has gotten me even more convinced that keeping a game like The Sims 4 going for so long must be really tough for the programmers.
You can read more about the bug over at Answer HQ.