Writer PDF font embedding doesn't work for BoD

Even if I use “File > Properties > Fonts > Embed fonts in the document” and chose “Archive PDF/A-1a (ISO 19005-1)” in the PDF export dialogue I still end up with totally messed up text when checking the result in the Book on Demand upload PDF print preview. (No matter what options I choose the exported PDF looks always fine on my Debian 9 system and when I check the “Properties” of the generated PDF it shows me a nice list of all used fonts and that all used characters should be embedded.)

I’ve tried various PDF options and even the print into file option via the printer dialogue. Nevertheless, I always get only totally wrong strange characters in the wrong font when checking the exported file in the BoD web interface. It could be a browser issue but at the same time the cover file I exported from Scribus works just fine instantly. So I’m wondering if there actually is a reliable way to export a pre-print ready file from LibreOffice Writer after all.

If I can’t manage to create a reliable file from LibreOffice I would need to layout the whole book in Scribus again and I would like to avoid that.

I found out that BoD does actually alter the PDF file in some way since I can open my original file just fine with Firefox but if I download the file the web interface of BoD displays I get a file that my browser can not interpret properly any more even if my pdf viewer still has no problems with it.