[BetterRed]

@mwlucas - curious, what did you use to create the DOCX - Word, LO Writer, Wordperfect… etc.

I looked at your sample DOCX with Word, the one thing I noticed was that the paragraphs with 'tight' line spacing have styling within them — e.g.filename and Users and Hosts… which converts to something like <span class=text4>blah blah</span>.

Is that true for all the paragraphs that are badly formed on Apple.

If you use the same styling (template) in multiple manuscripts you might want to consider using the Mammoth DOCX to EPUB converter rather than calibre… there's a Sigil plugin called DOCX Input which is a shim to Mammoth. It provides a mechanism to map DOCX styles directly to CSS entries… takes a bit of effort to set up, but providing it is reusable its worth doing.

BR