Sims suddenly appearing in my penthouse

Beyond locking the doors and the private lot lot trait, you could try that new send home and disallow visitors thing. Send home should be on the Sim and the disallow visitors should be on the front door. I have had mixed results, but it was mainly me trying to send paparazzi and stans home from Get Famous. It is worth a shot.

Otherwise it is probably a bug you will have to report.