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I'm not going to read through all of the posted comments, but I want to say that I've been having this exact issue (and so have many commenters on reddit, and I've seen it on multiple sim-youtubers' games as well). I've seen comments list all these "tips and tricks" for how to try to prevent or fix this issue.
In my opinion, it's simply low needs that lead the roommies to complain and then leave. I've had 100% success rate of my roommies saying "thank you for fixing the problem" and not leaving when I've cheated their needs to full using "make happy." I feel this is simply a product of a problem of the game's imperfect autonomy paired with some of the behaviors innate in the roommate (coming and going strangely, only sleeping when your sim does). I feel a game fix could be to change the decay rate of a roommate's needs to like.... incredibly slowly. Because the roommates rarely get to finish a need-fulfilling action between the player bopping around on different lots, interrupting, etc., plus the roommates are rarely given the time to do autonomous needs things when they're running out of the house for no reason and coming back with food 5 times a day, for example.
Sunday - last edited Sunday
@crinrictI didn't see an answer in this thread but I found out what the problem is. Any bed other than a single bed (other than for your sims):
1- will not allow the game to let you post for roommates
2- if you buy single beds for each person it will work
3- if you try to hide the single beds in the basement and assign the roomies to sleep in the doubles they wont.
Therefore their needs for sleep get Insane and they move out pissed off you haven't provided them what they need (ie. Beds to sleep in)
If you only have doubles, even with a single sim, you will have all these issues.
EA needs to check the double bed system (attached to universities pack maybe?) and see what the coding issue is for double beds and roommates.
Workaround.... buy only single beds for roommates