Seems to be related (or the same) as this bug I reported back in January 2019 where no lobbies will show up if ‘Show lobbies’ is set to ‘Joinable’. Setting it to ‘All’ will show lobbies but they all state ‘Difficulty: Unassigned’ and are unjoinable.
According to @FatsharkJulia this is a known issue and possible workarounds are:
- Restarting Steam
- Restarting your PC
- Clearing Steam’s download cache
- Changing Steam’s download region