By default, we display the Cisco DN's and paired the name you have associated to it in whatever you have defined as the Name Source.
This is usually LDAP or Cisco End Users
If the source for that name disappears, and we have nothing to replace it with, we will display what we last knew.
Normally we would fall back to the Alerting Name on the DN. If that is field blank, we have nothing to show, so we show what we were last given.
If that DN is no longer used, you can remove it from CUCM and the entry will go away.
If you want to retain the DN for future use, change the Alerting Name on that DN to something descriptive such as Unused DN or Available Cubicle.
Then re-run the Purge BOC AD Cache followed by Full LDAP Build and Update Names
Then relaunch the client and test again.

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