Thanks for reporting this and figuring out what the issue is.
Looks like when the forum post for the blog post is imported it doesn’t grab the image URLs so they get dropped. According to Jeff Atwood (one of the devs behind discource forum engine):
It uses the readability algorithm to find the core content on the page. This isn’t an exact science.
(from here: https://meta.discourse.org/t/embeds-stripped-when-show-full-post-button-is-clicked/104025)
Best work around at the moment is to click the “original entry” link instead of the “show full post” button.
Update: I think I figured out how to remove the Show Full Post button, so problem should be solved.