There were two people in the room, me and my brother
Ok, thanks for the information.
So, it sounds like your brother's game either crashes (i.e. becomes unresponsive or unexpectedly closes or shuts down and needs to be restarted) or he disconnects (his internet dips for a moment and he loses connection with you) causing the online game to end and since you're the last player still connected, you 'win', and are probably sent back to the online room lobby, where the settings and players are listed. Does this theory sound correct?
Also, if your brother's game is indeed crashing, I'm not sure why. I don't have a Chromebook to recreate the issue and I've never seen the game crash on my devices before.