Unfortunately YouTube & YouTube Music are not the same thing, although currently Google do not provide a way to access YouTube Music data except though YouTube (or more specifically the “YouTube Data API”).
What does this all mean?
It means that sometimes you won’t be able to see some of your YouTube Music playlists in Playlisty. And sometimes you will be able to see them but they take hours or days to appear after you create them.
It also means that very frequently when you try to import a YouTube Music playlist in Playlisty it will not be able to match some or many of the tracks.
This is because much of the data about your tracks (e.g. the artist) gets stripped off as it goes through YouTube because, for example, “artist” is not a data field for YouTube videos.
We really hope that Google fix this soon.