family member cannot add favourites that are already on my favourite list

Long title, sorry, my daughter cannot add songs to her private favourites playlist if those song are on my private favourite playlist.

We have different accounts on different devices 

We've had a family plan for years, this is a new problem.

If she looks at a song that's on my favourite list the heart is illuminated as if it's on her favourites, but it isn't and she cannot add it to hers. She also cannot un-favourite it.