Matchmaking problem for event modes

In the event mode, the lobby cannot find more persons if one, two or three people quit in the middle of queuing. (It more happens when only one person does)

This problem also happened when Top Gun Gauntlet mode was released.

The attched screenshot shows that the lobby is stuck at 15/16 for minutes. In the middle of filling the lobby, one person exited, and the lobby got bugged. Eventually, everyone quit to find the working lobbies.

Investigate and fix this problem.