What does this mean?
Headings in a PDF should follow the sequence Heading 1 -> Heading 2 -> Heading 3 without skipping intermediate headings.
It is acceptable to 'go back' to previous styles/headings but the sequence should then continue without missing headings.More help
How close this website is to fixing this issue.
This is probably because we're still working on this. Take a look at another issue.