You say you created it in Calibre. Was Indesign a part of the process at some point? When exporting from InDesign uncheck 'Include Embeddable Fonts'.
As Paul explains in this post Adobe's method of embedding fonts won't pass validation...
https://www.mobileread.com/forums/sho...46&postcount=4
Not sure how you'd undo it after generating the epub, but I'm sure it's possible. Need to poke around the file though to figure it out.