In my turn-based app, existing matches created with a newer version (5.1.2) are not showing up in the MatchMaker ViewController (or even when I call GKTurnBasedMatch loadMatchesWithCompletionHandler) on a device running version 5.1. (Newer version is run from Xcode download to device).
Note that in AppStoreConnect, I have the earliest compatible version set at 4.5.
This was not a problem before using the (now mandatory) SDK 26 (Xcode 26.4 to be exact).