My DOCX export comes out as plain single-spaced text with no formatting, even though I chose a template and trim size. Why?

This was a limitation, fixed in version 1.9.7. Earlier versions applied only the page size and a font name to the Word file, and that font was usually not installed in Word, so Word fell back to its defaults. Since 1.9.7 the DOCX follows the book's design: a font Word has, the template's text size and line spacing, justified paragraphs, first-line indents or paragraph spacing depending on the template, chapter openers with the chapter kicker, and proper heading styles. Update and export again; the manuscript itself does not need to change. This applies to imported manuscripts and generated books alike.