I'm now playing another household through university, and they do not live in the dorm, so it's perfectly sensible and correct for them to get bills, but something that happened with them made me realize another thing to mention about that bill my dorm twins got. Both times happened after I exited Manage Households where I had been editing university townies.
So, the twins that lived in the Drake Hall dorm got their weird bill pop-up immediately after I had been editing university townies in Manage Households CAS and loaded back to them. The bills did not arrive through mail, it was a San Myshuno type of pop-up with a guy's (landlord?) head icon.
The family who should also get bills since they live in a normal house got the bills pop-up the same exact way/moment: I had been editing university townies in Manage Households CAS and then loaded back to the family, the first thing that happens is a bills pop-up.
Could be a coincidence, but thought to mention in case anyone is trying to reproduce the bug.