I was wondering if you could give me some advice on how the sequence is on your end, because when I use matchmaking I have troubles getting the clients (mirror) started. Sometimes it works and sometimes the Mirror Start.Client function gets not listened to (I assume).
I am using Mirror and the PlayfabMultiplayerAgentApi and implemented your example on github.
In my case both players start matchmaking and get a match. Server then turns active in the GameManager. The Mirror NetworkServer starts listening as soon as the MPAgent does the OnServerActive Callback. (Like in your example).
When does the the playfab server turns active. After the getmatch result? I assume that there is a sequence / timing issue and I don't know where exactly.