Make sure numbering is off on paragraph style for a heading.
Create a new custom paragraph style for heading, setting desired Level. Never use the built-in Heading1, Heading2, etc. Create new styles and don't use Inherit from (that's only reliable in odt or Export to PDF, not Save As docx). Each style should be complete.
Never Ever open/edit the EXTRA Save As docx in LO. Only edit the odt file.
Add the docx to Calibre and make an epub2.
Make sure it's set to make a new file at each heading that should be a page break.
Check TOC.
Convert the epub to mobi. We use dual mobi for Smashwords and azw3 for local Kindles.
Firmware from 3.4.3 and later works with azw3.
https://www.amazon.com/gp/help/custo...MQC26VQQMM8XSW
Only ancient kindles before KK3 (but also DXG) need mobi.
You can't upload mobi to Amazon KDP. That wants epub2.