The issue happens when HTML in the following form gets posted to Discourse. It’s most likely to occur when a topic is posted to Discourse via the API:
<p><img src="remote-image-domain/..."/></p>
Any outer tags around the image tag will cause the issue, for example <figure><img src="remote-image-domain/..."/></figure>
When discourse attempts to download the remote image, the following markdown would be generated for the first example:
<p></p>
This will create a broken image. It can be fixed manually be editing the Discourse post to:
<p>
</p>
but just preventing Discourse from downloading the remote image with the disabled image download domains site setting is an easier way to fix it.
For posts published from WP Discourse with the Block Editor, the plugin attempts to fix the issue by processing the post with the following code before publishing it to Discourse:
https://github.com/discourse/wp-discourse/blob/master/lib/template-functions.php#L197-L230
It might be possible to implement a similar fix with for the Classic Editor, but with the Classic Editor the WordPress parse_blocks function isn’t available, so the fix would be more complex. My hope is that the issue can eventually be taken care of with changes to the core Discourse code.