question

What the PlayFab matchmaker does is to find the first available slot in a game session that matches the parameters passed in, reserve that slot for the player, and return the info on it. If you're also specifying a statistic, the available matching slots will be ordered by that stat, and the closest match returned. And finally, if you're also using the "start new if none found" flag, it'll start a new game instance if there aren't any slots available that match.

The main point though, is that it's not a queued matchmaker - it immediately grabs a slot and returns it. If that server doesn't use RedeemMatchmakerTicket to say "this user just joined my session" within two minutes, that slot will be freed up. To enable what you describe, you would need to have either client code logic that says "if I join a game that doesn't have other players within X amount of time, leave that server and try to matchmake again", or else server logic that kicks the player out if no one joins.

But realistically, if you have a non-trivial number of players, you should really only run into this if you have bucketized your players too much. The case of someone attacking your title is technically possible, but it would only work if that person logged into many unique PlayFab accounts, all from different IP Addresses, and used them to matchmake. Since most titles using dedicated servers are able to support thousands of players at a time, that would take an extraordinary amount of effort.