I’ve made a build of the release branch from source, and when using this version of the engine I am unable to find any sessions. None of my session code has changed between the launcher and engine version, so something is amiss here. Using a launcher version of the engine results in this system working fine.
For a test project, you can use either ShooterGame or Multiplayer Shootout. Build a source version of 4.20 from the release branch and publish it to steam with a valid AppID. If one player hosts a match (via listen server), another player will NOT be able to find that match.
Forum thread is here, but this hasn’t yet solved my problem:
https://forums.unrealengine.com/community/general-discussion/1497867-steam-online-subsystem-not-finding-server-with-find-sessions-with-4-20