This is actually an issue that has been identified with other customers in the past. This tends to be a result of duplicate device records in the database. In order to properly identify what records are duplicated, we would need for you to run a query against the MDM database and send us the results. We can then identify which records are duplicated and send you the appropriate query to resolve the issue. Once the duplicates are removed, your users will again re-appear in the user grid. This is only a display issue, as you stated, the devices should continue to sync without any problem.

I would recommend opening an SR with the results of the following query copied into a spreadsheet. This will tell us what records are duplicated and we can get you the appropriate query to clean them up.

select * from iosmodellookup