I think I know what you mean. This happens when you install Magic Dosbox to the “public” location, it stores all its data in /.MagicBox, which is indeed a hidden folder. You can access it with proper file manager apps, but the internal Android file picker can’t do this.
Should work alright if you install to the private location (Android/data/), but personally, I don’t do that for fear of deleting all my games on accident.
So it’s not really broken, but kinda irritating. I’ll pass it on. ?