yeah, it can be confusing at times and there are complaints about the functionality of the UI. It is missing a lot of “common sense” and “basic” features and it doesn’t really make sense why. But it becomes much more intuitive to use after some time with it and makes more sense. Not for everyone though and that is ok too. Hopefully the new upcoming version will fix some of these things.
I think the reason the songs in your library don’t behave the same as the ones in the playlist is because the songs in the playlist were there before the song was added to your library and is therefore still linked to Tidal so it is treated as such. I think if you were to make a playlist from the songs in your library, they would behave as songs in your library and not how they behave when you first got them from Tidal. I think if you were to add a song from Tidal to your library and then add it to a playlist, you would be able to edit it and heart it and stuff. I’ll try it on my end too