Fixed the video for ya. If you post the full youtube URL on it’s own line by itself (no link or anything) it will embed properly. Apparently the shortened url’s don’t work.
I use the share button from the YouTube app. Copy the link and cut it into a New Message creation. Sometimes it will turn it into a hyperlink automatically.
I noticed I have had an issue posting tweets lately as well.
Maybe I should try the app again. I removed it a few months back.