I've just run through the extraction process now and tested both the mobi and the azw3 output files in the Kindle Previewer.
Strangely the mobi file is behaving perfectly while the azw3 seems to have lost some of the styles - or that's how it appears in the previewer. Some text I had in italics (using styles) doesn't show as italics in the azw3 file, and the scene break "* * *" which was centred using a style is left justified. And yet it all shows fine in the mobi file.
Given that all my testing had been good before, I must assume it was always looking only at the mobi part of the file. Now I'm left wondering if there is a problem with my files, of if this is just the nature of the previewer. I'm using previewer v2.9 and see there's a v2.92, I'll give that a go.
ETA: Whoops
- it helps if you read the fine print, or the FAQ anyway. Got to select Kindle Fire or later to view azw3, and that works fine. Both extracted files working great.
Thanks heaps for your help.