I convert wordprocessor to epub2 first. Some people swear by Sigil, I use Calibre via docx. The Kindle Creator is stupidly limited in styles for headings and breaks.
I then upload epub2 to Amazon when I have checked that dual mobi and azw3/KF8 are OK.
This seems foolproof. Otherwise I was getting epub errors that Kindles don't care about and seeing font not quite right on first paragraph style on old mobi and KF8.
I can use the daisy epub check if in doubt. Also Smashwords picks up errors that Amazon ignores.
I found the KDP viewer a waste of time.
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