Font substitution on web

Uploading PDFs that have embedded fonts to
https://issuu.com
resulted font substitution in some characters and plenty of defects.

Same files were tested on the competitor of ISSUU
https://www.scribd.com all worked fine no problem!! also when shared with other platforms for print the output is fine;
Just wondering if any of this forum participants experienced similar troubles by uploading to ISSUU website … I couldn’t get an answer from the ISSUU customer support.