I'm delight to know that writer2xhtml solved your problem. I long ago gave up on the auto-export from Word processing programs to html. Instead, I paste the Word doc or Open Office doc (I believe that's the same as Libre) into word2cleanhtml.com online.
Does writer2xhtml work on Open Office as well?
I have seen many warnings that mobi files created with Calibre can and have been rejected by Amazon's KDP conversion. In contrast, I have had very good results uploading Sigil-generated epubs to the KDP.