Matchmaker is not backfilling

I want players to be able to join mid-game, but they just allocate a new server.
I’m already calling ApproveBackfillTicketAsync() every second to approve new requests and keep the service active.

I only have one queue with a single pool.
This is my Matchmaker configuration:

{
  "Name": "AdventureWorld",
  "MatchDefinition": {
    "Teams": [
      {
        "Name": "UserTeam",
        "TeamCount": {
          "Min": 1,
          "Max": 1
        },
        "PlayerCount": {
          "Min": 1,
          "Max": 4
        },
        "TeamRules": []
      }
    ],
    "MatchRules": []
  },
  "BackfillEnabled": true
}