This challenge step?
It isn’t a bug, overflow: hidden creates a new block formatting context. This thread has some more info even though it is about a different challenge.
I’m not sure I understand why it is done this way. I would avoid using overflow hidden unless you are trying to actually hide an overflow.
Just skimming this challenge I don’t think it is that great, I find both the HTML and CSS questionable. I question it even teaching typography much to be honest.
I might try and update it at some point. I would probably end up rewriting it completely and that is a lot of work.