Have you tried this suggestion from the plugin documentation:
If the
embed truncate
site setting is enabled on your site (it is enabled by default), feed topics will only display an excerpt of the feed post. Users can view the full post by clicking the “Show Full Post” button that is displayed below the excerpt. Discourse will then scrape the topic’s embed URL to get the full post content. If the full post content is not being correctly retrieved, you can fix the issue by configuring theembed whitelist selector
site setting. See Configure the Allowed Embed Selectors Setting for details about that setting.
Though I will note the whitelist setting is now called allowed embed selectors
. I’ve also had difficulty getting images from Substack to embed properly when using Discourse as an RSS reader. At least now I get a broken image that I can click through to, which is better than nothing.