It seems like the RSS detection feature has an issue with Youtube playlists which contain an underscore within the feed URL. For example, when detecting the feed for the following playlist, the portion at the end starting with the underscore is excluded when choosing "Subscribe > Current playlist feed".

https://www.youtube.com/playlist?list=PL6pOqn4K529Kw0KSfKS9FO487fpiXx_6H

Results in:

https://www.youtube.com/feeds/videos.xml?playlist_id=PL6pOqn4K529Kw0KSfKS9FO487fpiXx