On servers that have Gameshark installed and enabled, the Gameshark UI flickers for clients. For example, if there's a boss nearby, the UI will be grey and say "searching..." for a few seconds, then flash blue for a tick, with a line drawn towards the boss.
This was tested on a server with a freshly generated Gameshark config, the only change was changing "enabled" to true.
Waterdude mentioned in the Pixelmon Servers Discord that there were reports that this bug only happens in worlds generated in 8.3.0, but I was able to reproduce this in a newly created 8.3.1 world as well.
In Singleplayer, the UI works as expected (as long as Gameshark is enabled in the client's configs)