The iframe for ISSUU embed is very little in the front end, even though in the back end it looks like normal size. No matter how the embed is added to a page, either by directly adding a URL or by using the ISSUU embed, the frame will not render well in the front end. 

Also, there is a lot of space around the iframe. 

It happens in posts and pages, both templates. 

Eg:

Post: https://k8s.p4.greenpeace.org/gutenberg/story/24529/testing-issuu-embed/

Page - default: https://k8s.p4.greenpeace.org/gutenberg/testing-issuu-embed-default-template/

Page - evergreen: https://k8s.p4.greenpeace.org/gutenberg/testing-issuu-embed-evergreen-template/