The short answer: don’t use .doc or .rtf export. To get the styling in Word document that you want, use export templates that incorporate CSS styles in HTML exports. The CSS defines the styling for header levels, H1, H2, etc. Export the note(s) as an HTML file and import that to Word. The headers you defined in the HTML will be recognized by Word, and the CSS styles will be saved as Word Styles. Re-save the document from Word as .docx. You’re done
I do this over and over and it works very well.
You can download from the link below a zip file that contains a sample Tinderbox document with working examples of the templates you need, and a container to export into a simple document. Also included are the exported .html is included (open in Word); and the .docx I saved from that .html file.