When using Calibre to convert to AZW3 format, under the Look & Feel Options--in the middle of the page there's a tab labeled "Filter Style Information". On this tab, there's an option to completely remove fonts. This should remove all the font information coded in the book and the Kindle default fonts should work as expected. Note--this will strip all the font information, so if the book contains special fonts for different parts of text, etc these will all be removed.
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