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More update. What I'm seeing is that the DeDRM'ed AZW3 file had decrypted fonts in it (as seen with Explode book).
Calibre Convert is encrypting them when converting to epub. I don't know why it would do that, but I can see it as one of the last steps in the conversion process. I've not found any setting to tell Calibre to not do that. Nor do I see anything in the exploded AZW3 to suggest it should.
In fact, I went and downloaded a TTF font file, added it to an AZW3 file that didn't previously have any fonts and when I converted to epub, Convert encrypted that font.
I even took an epub with unencrypted font file and did epub->epub convert. Output epub also had encrypted font file.
So it looks to me like Calibre is encrypting all embedded fonts when converting to epub. Is that intended behavior, I wonder?
(FYI, there's a checkbox setting in DeDRM plugin for 'Deobfuscate EPUB Fonts'. I assume it's on by default.)
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