Consider giving node-admins (at least) "view contacts" access in X-nonmembers-group. Then users without group membership won´t just disappear. That´s how I managed to get rid of calls about this "annoying feature". "View contacts" doesn´t of course give node-admins rights to place the user back into a group but they can at least find the user and figure out what happened = less confusing.

Also, if a local admin runs a contact search with selecting "contacts created by you", the lost users should appear and be modifiable.

Though as mentioned above, an automatic import may resign manually created users from imported groups. This depends on the import and its settings, however.

I guess you where refering to my posting above, it's not an issue about getting dropped out of imported groups, its about when you make a new contact in a manual group. During the creating phase there is a risk of loosing the tick in front of the group where you created the contact from.