Once we were working on an issue in which username mapping was not working properly. Whenever we add any advance map entry, it gets disappear once we re-open the MsNFS GUI or run the command mapadmin list -all.
Same thing happen while adding the map using command line.
Upon troubleshooting, we found that HKLM à System à CurrentControlSet à Services à Mapsvc à CurrentVersion:
registry keys are populated but nothing comes when we run mapadmin command or in username mapping GUI.
We delete the entries in those registry keys, restart username mapping and create another map but still no luck.
Then we confirm username mapping service to start with domain admin credentials.
Restart username mapping and found that things are working fine!