We have added new patron categories and specified that some categories are now only for certain libraries. If a patron has one of the now restricted categories, when you edit the patron, they are automatically changed to the first unrestricted category in the list. This changes the patron category unexpectedly when editing the patron. If a patron is edited, and the category they are in is no longer valid, it should default to a blank selection and force you to choose a new category.
Hm, this should have been fixed by Bug 31422 - Library limitations might cause data loss when editing patrons It allows you to keep the current patron category, even if you can't pick it for a new patron. *** This bug has been marked as a duplicate of bug 31422 ***