Just attempted to start a game, another player joined and a few moments later MY start game button vanished.
I asked him if he still had his, and he said "i pressed mine already, it says i voted to start the game."

i left the game and started again, got a new lobby. he joined again and i pressed the start button, and received the same message, he was able to press his button and the game began.

I wonder if the person joining pressing the button is what caused my button to vanish.

having tried a few more games since, it appears that if the first person in the room presses the button it works to launch the game, but if anyone who joined later presses the start button it dissapears for the other players and the game wont begin.

Happened to me too, as soon as they pressed ready my button disappeared and I couldn't start. I think the host has to be the last to press ready, or else the button disappears for everyone else.

I've been experiencing the opposite. it seems to me that the first player in the room (the host) has to be the first to press the button.