[2021] When in lobby, other players not showing

Are you all using the Steam version? Or is anyone on the MS store/gamepass edition?

What version and checksum shows in the MP lobby screen for you?  

That is correct, so I can only assume you have a firewall issue. Have all players opt into the Crossplay test branch, now in the Steam Betas tab for this game. Do you get the same problems?  

Using a different client does not affect the game's MP functions at all. What it does do is make a second installation of the game, so if the problem is a corrupted installation under Steam that could conceivably help.