I figured out the issue. I was not registering my GKLocalPlayerListener with the GKLocalPlayer register function (https://developer.apple.com/documentation/gamekit/gklocalplayer/1515393-register). I also posted a suggestion to Apple's feedback to update the documentation with this information (https://developer.apple.com/documentation/gamekit/finding_multiple_players_for_a_game).

Did you ever figure this out? I'm having a very similar issue except I can't even get the player:didAccept to happen on player 2's device....