I've had the roommates join the household on loading a save glitch. It happened most often when I only had the patch but hadn't installed University. Once university was installed a more robust roommate management system was available in the home world. I don't think there's even the option to enable roommate services if you've only got the patch. I didn't play it much before installing the expansion, but it seemed the patch only roommates were inherently glitchy (in terms of strange joining/leaving household ways, not corrupting save file ways).
Darstan, whenever that happened to my family I'd kick the roommate out, then immediately invite them back over and ask them to be a roommate again. It's a bit of a drag, but it works. If one of your sims has the legendary host LTR they can throw a party and only invite the kicked out roommate - that guarantees the sim will show up and you'll be able to ask them to room with the family once more!