Switching servers when logging in?

[Mythril]

Hi there.
There's this problem I have with the servers. My friend and I play perfectly fine together. However, his brother, whenever he logs in, the server automatically changes. So say, he chooses Vachira. Logs in. Can't find us. So he logs out, and wow, the server is the Angama Ocean.

Is this a bug? Or is there a setting to stop this?

Thanks.