Not sure if this is the same problem you experienced, nor do I know the defect number, but we had a similar issue. Here is the TAC response:
The backend engineering team updated back that after further investigation the "ghost" users from the last example joined the meeting with emails "cxxxx.sxxx@cxxx.com" and "jxxx.sxxx@cxxx.com". They confirmed these users did show different display names on endpoint devices than from desktop clients. On the endpoint devices, the users’ names were displayed as “Christopher” and “Jennifer”. But then on the desktop clients, the users’ names were displayed as "Chris Sxxxx" and "Jennifer Sxxxx".
After additional analysis the backend team identified the issue was due to a conflict of user values that caused the backend side to replace the names given to Webex when the users joined. To resolve this issue the backend team applied a fix in version 42.3 which had been deployed to the cluster that site cvs.webex.com is on (cluster M).
