I can’t remember if it was when Steam changed their Friend list UI, it affected many games.
Sometimes when a player’s profile was private, nobody could join them (I think? or maybe it was the opposite).
But in your case you were able to connect while in the same local network, so maybe it’s not that.
I’m not sure how does the game choose the port (that’s probably handled by the Steam API). Perhaps you can try forcing to use that port you forwarded like for non-Steam users (not sure if it would work or if the Steam API would overwrite it).
These go in the user_settings.json at the same level than user_id. Later if the client restarts the game to play singleplayer, the game will attempt to connect to the host so it might take a bit until it fails to connect and brings him to the main menu, so he might want to remove these settings.