Something wrong in your source docx.
I've converted docx to epub from LibreOffice Writer over 1000 times for over 10 years. Edits in odt and an extra SaveAs in docx. DON'T ever edit the docx in LO again, unless you check all styles, anchors, links, headings etc and save/edit as ODT, because it converts docx.
Google docs exported to docx need reformatted / styled in Word 2007 or later or LO Writer 5.x or later. I'm at 7.x something.
Anything saved locally as docx from ANY Web based editor needs fixed in Word 2007 or later or LO Writer 5.x or later.
The paragraph styles map to CSS for<p tags and character styles map to CSS for <span tags.
Make sure there is only ONE page style. Make it A5 with small margins, no headers, footers or page numbers. Edit a final proofed copy with multiple page styles with true size, headers, footers, page numbers, outer & inner margins & registration etc for paper or POD (export PDF from wordprocessor). An ebook and paper ideally might use different fonts, especially for body text. Make sure ebook version is free fonts with fallbacks.