Actually, it happens in PT/Libs too. There is a fix for this though.
Go to your Options > Gameplay > Default Channel > Private
Now it will always be private and if you wanted people to join, you can click the "Public" button before starting a dungeon.
However, this also has a bug when it comes to Event Dungeons. If you do a run of Twilight Forest and try to restart it, you'll get an error that says you can't join the same dungeon, to fix this just go back to options and set the default channel back to Public.
Now you can freely repeat your Event Dungeons again. It's annoying but that's BugWarz for you.