SharePoint 2016 Server User Profile with Custom Authentication, losing email address

Question

I've a SP2016 server that uses FBA for user accounts. Currently, there is a PowerShell script that synchs user information up with a user database.

An issue observed is that sometime user lose their email address (and first/last name for that matter). I've increased the frequency of the synchronization in an attempt to reduces cases where a user misses a notification.

I've investigated the User Profile Manager, but currently I have it disabled. Also, I've investigated the possibility of setting up MIM with UPM, but I'm not sure that fixes the problem and it is a significant investment of my time to get that to work if it is indeed possible without LDAP (maybe I don't understand MIM?).

Do any of you have ideas about what might remove user information? I can only assume a app service is doing it, but which one?