My guess is that if the player would forfeit on time in such a case, the sealed move is opened and if that move ended the game, stalemate, checkmate then that move would stand.
That said, I don't think very many tourneys have such a situation occur anymore. The only situation where it might is in the case of very strict round starts where a previous round is still running and would delay the next round unacceptably.
Law E.7:
"The envelope shall be opened only when the player who must reply to the sealed move is present."
But Law E.10:
"E.10. Any player who arrives at the chessboard after the default time shall lose the game unless the arbiter decides otherwise. However, if the sealed move resulted in the conclusion of the game, that conclusion shall still apply."
So, suppose that the player who has to reply to the sealed move fails to show up.
The envelope must not be opened until the player who has to reply shows up. Which he does not.
Eventually, the remaining time of the player who had to reply runs out... at which point the player would lose.
Since E. 7 says that the envelope cannot be opened unless the player who has to reply does show up, can anyone ever open the envelope and discover that the game resulted in a draw (if the sealed move was a stalemate etc.) or indeed by win of the player who did not show up (if the sealed move was illegal etc.)?