When you click the “Show Full Post” button for an embedded post, Discourse attempts to retrieve the post’s content from its URL. That content is then cached for a period of 10 minutes. Depending on the embedded post’s markup, sometimes Discourse has trouble retrieving the post’s content. This can be fixed by configuring the “CSS selector” setting that’s found in the Crawler Settings section of your Admin / Customize / Embedding page. More details about this are here: Configure the Allowed Embed Selectors Setting.
For your case, I’m unsure why images and heading tags are being excluded and paragraph text is being included. Possibly you’ve already added some selectors to this setting and those selectors are excluding heading and image tags.