That's pretty normal with NPCs and even happens at times with played sims that are not in your current active household. The reason it happens is because the game uses sims outside the active household for other roles and puts them in situational outfits which you have no control over. So, your NPC roommate is probably one of those that keeps getting used for other things by the game. It could be something like the sim gets used for spirit day, debate team or a university mascot. They keep the clothes you gave them until they get changed into their situational outfit and when they get released from that role, they don't necessarily get changed back to what they were wearing before. Actually, I'm a bit surprised that it's not happening to more of your roommates.
There is probably not much more than what you've already done to deal with it. If you use mods, and specifically MCCC, you can change them back to the outfit you assigned them using the Dresser module. But, if you don't use mods, then there's pretty much nothing else you can do about it.
Hope this helps.
