This is build v1.1.2a since it says to include that in the bug reporting FAQ. No Cats and Dogs DLC.
This happened around year 25, low 600s population. It was also not during a raider attack, no major building going on (I guess I was converting all my dirt roads to stone roads), during the middle of the year (late summer), didn’t move any buildings, upgrade existing ones, or build new desirability lowering ones…can’t think of anything else circumstantial.
Randomly, my desirability started tanking in an area of my huge residential area for no reason. I think. Basically the exclamation mark popped on like 5 houses, people threatened to move out. They had dropped 3-5% or were in the middle of a bracket (which would mean their desirability dropped a lot or they just randomly got that status). As I started building parks and gardens and whatnot to boost it, a few more got the mark…then a few more…then a few more. As I’m scrambling, every time I unpause a bunch more get it. Then like 20 houses all go vacant. Desirability is awful in some spots, still good in others. Save the game to go back and look at an old save and see how much values dropped, then reloaded the buggy save. All the desiribility is back up and all the abandoned houses are back to being fully functional, max desirability mansions.
The couple abandoned houses I told the game to demolish before I saved randomly disappeared in the next minute or two. Pretty much entirely fixed itself just by saving and reloading.
Seems like an easy fix but I nearly rage quit the game and never came back to it. Being down 100+ housing capacity randomly within minutes and having desirability tanking all over the place was not fun. It’s been stable since but no idea what triggered it, sorry.