When I'm trying to join a server using the Steam browser protocol (e.g. steam://join/<ip>:<port>), it won't work and give me a "Server is not responding", where as the IP is exactly the same as where I click on "View game info" on a friend's name.

However, when using the Steam browser protocol in combination with the server's Query port (server port + 1), it does launch Arma 3's launcher.

I think that this is a mistake as the join should use the connection IP, not the query port