Apologies for late reply.
Yes, you are right. Whenever a user joins the room and if he is in the lobby then he will be automatically out of the lobby on the server.
The notification gets fired in if a user calls leaveLobby API or he/she joins a room.
Regarding the notification not fired when joins room, I have forwarded this to my development team to check and will get back to you asap on this.
Also, if your use-case can be achieved using a static room as of now then you can ahead and use it.
Static room and Lobby both have different behavior so its depend on your use-case that which you want to use lobby or Static room. However, you can use both or any of one based on your requirement.