I encountered this puzzling behavior. I know WHEN it happens, but I'd like to know WHY it happens
So I create Word docx with 3x3 matrix of three fonts and three versions - regular, italic and bold. I tick option to embed all fonts, no subsetting.
See first screenshot.
I let Calibre convert it to AZW3. I check it in Calibre Viewer - second screenshot, looks the same (fonts are there, italic is italic, bold is bold)
I upload it to my Kindle, inspect it with Bookerly. Third screenshot, italic is italic, bold is bold.
I switch to Publisher font. Fourth screenshot, Regular and Italic is okay, bold is Regular now!
When I tick option "Do Not Embed Common System Fonts", save DOCX, convert to AZW3 and upload it to Kindle, Arial and Times are Bold, Source is not.
And, when I don't embed any font to Word file, everything is correct on Kindle side.
So we now know WHEN this happens -> when Word embeds bold font to DOCX file, Calibre uses this embedded variant and this is then for some reason ignored on Kindle. When the font is not present in DOCX (no embedding at all, or no system font embedding), Calibre uses installed font from Windows and such works great on Kindle.
My question for people who understand fonts -> what does Word font embedding do to Bold fonts that Kindle does not like? And/or is there any option hidden anywhere which would let me use only font metadata from Word and not actual font files?