YouTube Music is an overlay of YouTube. That means when you are adding tracks to your favorites or to a playlist, Google will then do a rework to match to the YouTube Music catalog.
This "rework" feature could take some time, and that's why after a convert process of a playlist to YouTube Music you will see the tracklist not available for a couple of minutes (hours depending size of your playlist) and after that, one by one, tracks will be available again.
Just wait some minutes after the convert process has been done to let the time to Google to finalize your playlist.