Thank you for your quick reply. 
I have also tried another feed without strange markup.
Here is example from another feed:
https://grapee.jp/category/animal/feed
âEmbed Truncateâ Disable:
The broken image is a known problem caused by the <p> tag. It can be fix manually following the instruction from this post .
But the main problem is, it only show a very short description from the RSS feed, not the whole content from the web page.
âEmbed Truncateâ Enable:
It doesnât show any content, other than the original source link.
âShow Full Postâ Button Pressed:
It display the correct content from web page following the whitelist rule.
Do you think it is possible to show the full post content by default, without pressing the âShow Full Postâ Button? 
Or do you think it is possible to let Discourse scrapes the page content from the web page that is associated with the feed item all the time?