The package highlight-refontificaionrefontification visualizes how font-lock refontifies a buffer. In this case, it starts at the line where the point is.
One way to handle this is to add a function to font-lock-extend-region-functions
to expand the region to include the full heredoc comment.
One thing that is curious is that the highlighting becomes correct when pressing return
but not when evaluating (insert "\n")
, something I have no explanation for.