Hi @Patrick_Riecke and @anonwriter, thanks for your description of this problem. We’ve been able to replicate the issue and have filed a bug report which our team is now investigating. Until a fix has been released to production, we’ve found a workaround that may be helpful. It’s possible to use Calibre to convert your EPUB to an EPUB (rather than to MOBI), as strange as that sounds. The outputted EPUB file can be opened in Kindle Previewer or uploaded to KDP without encountering this bug.