Same problem here. I get the feed and the thumbnails are visible in the article list but when I open an article, it shows just the description from the video and no thumbnail.
I looked at the extra html heading thingy and the examples provided elsewhere on this forum and I think the problem is that the code defaults to a picture called 0.jpg when Youtube V3 rss feed links the picture as hqdefault.jpg. Also, in the html header sample the first part of the address always uses
https://i1.ytimg.com/, but for some videos Youtube also uses
https://i2.ytimg.com/ and probably other numbers as well.