I have done the challenge twice and it still is listed as not completed.
Did you change the anchor tag to
<a href="#footer">Jump to Bottom</a> and add an id to the footer like
<footer id="footer">....? In my case, it worked perfectly.
Yes, it says challenge completed but when I go to the overview it was marked as not completed. However I tried it again after waiting a while, and it worked fine.
Yes, it happens. I experienced the same in another lesson.