I have this happen every single time I make a game, or my friend makes a game. Nobody can join our "public" games unless invited. This also happens to me when others create a game and tell me to simply join. I almost always can't and have to be invited, even though other players are already in the game without being invited. Both of these bugs seem to only affect some players and not others. It happens with about 90% of games made by others, and always between people who are confirmed to have this bug. People who don't have it usually can join my public games with no problem even without me having to perform the below workarounds.
There are two workarounds to create a truly public game.
1. Create a Play Solo game then use portrait options to change the game type manually after host loads in.
2. Create Public Party then toggle the portrait options to anything then back to what you want. The settings actually take and people can join normally, including friends affected by the same bug and strangers too if game is fully open.
This is really annoying and has been happening to me since the party create options got changed. I just kept forgetting to report it.
The party seems to be joinable for me after the player finishes loading the map under these conditions.
It remains unjoinable while they are still loading the map unless an invite is sent.