It's because they are cached that way. It's probably only one or two weeks of posts affected- now that it's working properly the old posts will get fixed in time.
I don't know of any way to speed this up other than to manually edit and re-save each post.
But in time it will get back to normal.
Also I was thinking about your youtu.be issue- I made a quick mod that will change youtu.be to youtube.com so it will always use the real youtube.com URL.
https://vborg.vbsupport.ru/showthread.php?t=267450