This is either a bug report or a feature request, please relocate/tag as needed:
When viewing an embedded YouTube video on third party sites the “Watch Later” and add to a playlist functionality is broken by default unless allowing all third party cookies.
There are optional exceptions in settings for social logins, I believe an exception should also be added for YouTube cookies. Even better let users decide to whitelist any third party cookie, perhaps even on a per-site basis like Privacy Badger. As it is users must allow all third party cookies if they want to interact with YouTube videos around the web, which is a nasty decision to have to make.