I want to say ahead of time I already have a TAC case open on this issue, I'm opening this up to see if anyone else is seeing similar behavior.
We recently upgraded from a 10.5 cluster to 11.5 including IM&P and CUC. We also upgraded our organization to the 12.1 jabber client, on this new client we've gotten quite a few complaints from users about incomplete users listing in persistent chat rooms. At first reports were that they could see some users in the room but not others, after doing some digging I've isolated the real behavior.
If user A joins a chatroom any user that is currently active in that room can see user A. If user B joins the room user A can see user B but user B cannot see user A or any user that had entered the room before them.
Now the really odd part is if you go back and install any 11.X or 10.X jabber client this behavior is not seen. I haven't been able to test this behavior in the 12.5 trial due to some legal red tape of accepting mandatory reporting.
Only happens in public rooms and running windows client. I just tested out 12.5 last week and the issue isn't present in the new version. I have a screenshot of the 2 different versions in a side by side comparison. 12.1 had only 2 users in the room but in the same room on 12.5 it had over 20 users. I can send that screenshot via email if you would like.
Please send me the snapshots to email@example.com.
Basically in public rooms, you can only see the participants who is opening a chat window of this room at the moment.
BTW, I assume you are using IM&P 11.5 SU5 as the server version.
If the behavior is different with this, it probably is a defect.