Hi! StartMatchmaking will match backfill tickets against all tickets submitted using StartMatchmaking. It's not necessary that the same ticket will be matched to the same game session it was previously a part of. What I'd look for as part of auto-backfill is if the open spot was matched to some valid player.
If you're using managed GameLift hosting you can potentially try using CreatePlayerSession to reserve an open slot for the dropped player before backfill finds another player for the slot.
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