When you first post a YouTube video to Facebook, Facebook makes a cached version (a copy) of your post. If you change something on YouTube, Facebook still shows the old version. But you can try forcing Facebook to update with this page

Just add your YouTube URL into the box and click Debug. It will give you a bunch of information – but you can ignore the report (even if it has errors.) Just try to post to YouTube again and see if it’s updated.

Screenshot from Facebook Debugger