I'm tempted to apply the following patch, as this problem falls under
the category of "inserting in an invisible region". I don't think it
breaks any non-interactive call of org-insert-heading, but this needs
further checking.
Also, note that the problem occurs only when there is no blank line
between the :END: and the following headline, so maybe the real fix
has to be within org-insert-heading...
--
Bastien