First of all, apologies because I was wrong above. It is the other way around: It refers to when a track was added to the library, not when it was added to the playlist.
I just checked my playlists:
- If the playlist is a copy of a Qobuz playlist, where I don’t have the tracks in my library, then the Date Added field is empty. This makes sense because I never added them to the library.
- If I created the playlist myself, I always use tracks from my library, and these are shown correctly (with the date of when the track was added to the library).
So if you never see dates, could it be that you simply are not using in-library tracks?
The behavior of using the added-to-library date makes it of course impossible to use this value for sorting the playlist by the added-to-playlist dates. I think there are open feature suggestion threads for this.
EDIT: There is at least this one but I am quite sure there must be others with more votes