Taking an ODF document written in version 5.2.7.2 of LibreOffice, I saved the document as a .docx file and viewed it in Microsoft Word 2016. Everything was the same in the .docx document except that underlining was lost. It wasn’t just the visual aspect that was lost: when I placed the text cursor into the middle of text I knew to be underlined in the original document the “U” button in the Word toolbar didn’t change to active.
Of note is the fact that italicized and bolded text were identical in the original ODF document and the new DOCX document, so the exporting of formatting in general is working.