Closing minecraft or disconnecting from the server will kick you from a match5 game, upon rejoining your opponent will be stuck in the game and their only option is to forfeit but you cannot forfeit the game while the opponent is offline so as far as I am aware you are stuck in Match5. 

Priority Fix: Medium

Which Server: Spawn

Reproduce: Players can join Match5 games with other players and then leave keeping the opponent stuck in the game then that would prevent others from playing.

Expected outcome: I expected the game to end if one player left.

Actual Outcome: The game does not end and the opponent is stuck.

Proposed fix: I am not sure how hard or easy this would be to add but when a player disconnects mid game in maybe 1 minute end the game and return the diamonds back to each player.