While Otto’s advice will help you to avoid the situation, the actual issue is exactly what you identified - in this situation Plex does not honor the lock setting. Whether or not your files/folders meet Plex’s requirements, when the lock is on, Plex agents must not alter that field, period.
This is a bug that was identified at least 4 years ago, and was reportedly fixed in PMS 1.4.3. So either it wasn’t actually fixed, or there has been a regression.
