About automatic backfill

Hello, I am trying to implement automatic backfill mechanism for my game and i have some issues with this. After i search on the forum, i get quite confused as well. So my setup is like that: i have a game session with a max of 4 players. I use flexmatch to place players to game sessions. When the player connects to the server and then disconnects from it, i can see player session status as completed. However, i cannot use this slot anymore, meaning that if 4 players get connected and then disconnected from the game session, for 5th player, gamelift opens a new game session. My initial assumption is automatic backfill handle these completed or terminated player sessions and place new players to these slots. My question is, is automatic backfill do that or i get it wrong? If automatic backfill does not cover this, how can i reuse these slots in a game session?