you cannot prevent this message from appearing in the server chat.
Whenever a user joins or switches to another channel, the server assigns the appropriate channel group to that user.
Since channels are independent from each other, the channel group has to be assigned again when entering a channel, and this action is logged in the server chat.
There is nothing to explain about this. The channel groups will be always assigned on every server when switching channels!
Your first screenshot shows us exactly the same message from your topic post. On your second screenshot you didn’t switch channels, so there is nothing in the server chat.
If I haven’t overlooked a screenshot here… and it really isn’t displayed on a server in the server chat, then it’s either third-party software or modified TeamSpeak servers. These are not supported and will be blocked accordingly.