Not sure if this is a bug or just the design of the platform... but whenever I want to post a Youtube video I insert it into the comment section - and I make an additional comment when I post the link... however when you post it, only the video shows in the news feed and not the comment that the person made when initially posting the link...
Either way if it's a bug or not, I think posting comments should show when initially sharing the video links... reason, is because I spent a couple minutes typing a comment out about a Youtube video I shared and only to see it was not posted with the video... I know on facebook you can post this way and the comments stay with the initial post...
Gary