Impossible to find Steam Sessions when using Source Build in 4.20

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:

Note - if I launch the game OUTSIDE of Steam (and ensure that the steam client is closed on both machines), the game will fallback to the IPNetDriver and I will be able to find sessions without any issues.

This is clearly an issue with the Steam Session Interface, which for some reason only occurs in a source engine build.