I have noticed this as well. Some are songs that I have had for years and some are Apple Music songs added in the recent past. They are not available to play on my playlist but when I search for them I can play them without issue. When I add them to the playlist again they are still unavailable.
I have restarted my device a few times (iPhone 13 Pro Max on latest iOS 16.1). This is also happening on my iMac using Apple Music (on Ventura 13.0)
Happens on old and new playlists - tried to create one and it will not allow me to add the song (tried "Off he goes" by Pearl Jam to no avail - plays when you search for it but not when you add to playlist)
Seems like a glitch rather than a content / location restriction as they play stand-alone but not when on a playlist.
Apple, please help! I have worked really hard to build a lot of playlists and would like to access them. We pay a decent amount monthly for this service - please fix this issue!