The reason this is happening is because the “Show Full Post” button on an embedded post in Discourse works by “re-crawling” the embedded webpage when the user clicks it. The HTML you see when you click that button is not being sent by the WP Discourse plugin when the post is published, it’s a fresh crawl of the relevant page.
Web crawling is always a bit hit and miss due to the wide variety of possible HTML structures (e.g. there are many different ways of displaying images in HTML). Because of this, this is a feature that is being considered for a possible improvement. See further
So there isn’t currently an easy solution. You can attempt to block and allow HTML selectors the crawler uses, however this is a tricky thing to get working in the way you want. See further
A simpler way of addressing this is to either hide the “Show Full Post” button, or override it so it links back to the original Wordpress post, using a custom theme component. When a site has a relatively complex post HTML structure involving images, like yours, I tend to advise these latter options.