I think I am having the exact problem that was described here: https://kriesi.at/support/topic/aspect-ratio-of-embedded-youtube-video/
YouTube videos work just fine in the advanced layout editor, but they are tall rectangles when I copy and paste the youtube link into a standard blog post.
The custom CSS shared in this post https://kriesi.at/support/topic/aspect-ratio-of-embedded-youtube-video/ is not working.
I am on version 4.6.3.1