This is from the documentation (http://helpx.adobe.com/acrobat/using/create-verify-pdf-accessibility.html):
Appropriate nesting This rule checks nested headings. When this check
fails, headings are not nested properly.
To fix the list structure:
Find the list in the Accessibility Checker panel by right-clicking
(Windows) or Ctrl-clicking (Mac OS) the failed element and choosing
Show in Tags Panel. Create elements, change the types of elements, or
rearrange existing elements by dragging them.
You may want to check again that there is nothing between your last heading element and this heading element that could potentially change the hierarchy. Is it possible that the nested heading is part of another element?
It is always possible that you came across a bug in Acrobat, but I would first rule out anything else.
Karl Heinz Kremer
PDF Acrobatics Without a Net
PDF Software Development, Training and More...