Problem with external hyperlinks in pdf exported from .odt files


I have two .odt files both with headings:
In document 1 I added an hyperlink with a target to an heading in document 2

In the .odt files this hyperlink works correctly: when clicking on the hyperlink in doc 1, doc 2 opens and scrolls down to the right heading.

Then I export the two odt files as pdf, selecting in the “Links”:
“Export bookmarks as named destination”
“convert document references to PDF targets”
" Export URLs relative to file system"

Now if in pdf-1 I click on the hyperlink, pdf-2 opens, but not in the wrong location.
It appears to open always at the beginning of the pdf without scrolling down to the required heading
With pdf-2 opened, the situation is unchanged: if I click in the hyperlink, pdf2 still does not scroll down to the right heading.

I tried both libreoffice version 5.1 and 5.4. Ubuntu 16.04 and Windows10
Either one of my setting is wrong, or this is a bug.

Any help is appreciated