I know that the TeamSpeak Server deletes clients with no group (i.e. the guest group) after some time (in my case it was exactly one month). This is normal and expected.
But it seems that the TeamSpeak Server also deletes clients out of the database if they had client rights set and even a query login (which gets deleted too). In my case I connected with a new identity “serverlist_client”, gave it some client permissions to list groups, etc. and created a query login for that user. Exactly one month later, the client was deleted and the query login too.
I fixed this by recreating the user and giving it some random group this time, but I still think that it is unexpected behavior that even clients with set permissions are cleared from the database (as long as they are in no group).