GameLift FlexMatch Matchmaking Configuration WITH_QUEUE placement timeout

I'm attempting to use GameLift Custom Server FlexMatch MatchMaking Configuration with a game session placement queue.

The Matchmaking Event Handler Lambda first receives a MatchmakingSearching event, then a PotentialMatchCreated event.

The Game Session Queue Placements Started event shows up in the Queue metrics section, but it will then timeout (after 5 minutes). The GameLift Game Session is being created, but the instance process' OnStartGameSession(GameSession gameSession) is NOT being called..

Can someone please help me figure out how to get the OnStartGameSession to be called in the fleet instance process?

Before attempting to use FlexMatch, I was able to start the placement manually and the game session was being created on the game server, and the game was playable, but I'm unable to get to that step using the automated FlexMatch matchmaking system.

Thank you.