Fix for Distorted Italic and Bold Text in EPUB Export from InDesign

Nothing jumps out except the format. A poor EPUB reader can be one problem, as can use of variable fonts. But as so often, FXL itself is a problematic format at best, with the problems multiplied when used for text-page output. It's also not InDesign's strongest feature.

One short answer is that if you must have print-replica pages, use PDF. It's what it's for and it works almost perfectly. EPUB is at its best with reflowable text content or a narrow range of "picture page" formats; exporting text pages to fixed layout is fraught with problems and usually unsatisfactory.

The Apple viewer is somewhat nonstandard but not bad. You might try viewing the exported file on a more standard viewer such as Thorium or Calibre; if it views correctly on those, or at least loses the font problem, it narrows things down.

The fonts seem okay as a choice (not variable or otherise nonstandard). But fonts in EPUB are... a whole other area of problem. If you're using anything but the base four fonts (regular, italic, bold, bolditalic) the format and reader may not be processing them well; you can't, for example, use alternates like light or semibold or black in reflowable EPUB. (Not easily and without faults, at least.)

The only start on a solution would be to make sure your use of styles (paragraph an character) is spotless and without any spot overrides or local formatting. No matter what else it is, EPUB is absolutely reliant on technical source perfection, and things you can get away with in print (or PDF) will blow up in a digital export.

But mostly... it's likely to just be FXL's limitations, unsuitability to text documents, and InDesign's indifferent export function. A switch to PDF (if you have to have fixed pages) or reflowable EPUB (if you must have EPUB) may be the only options.