I just hit this as well. For me, it only happens on playlists that contains only content I have uploaded.
When i ask Siri on my iPhone or HomePod play it Siri replies "Now Playing Playlist XXX", but nothing happens.
I found a surprising workaround. If you add any Apple Music song anywhere in the playlist, it starts working again. I have been adding Brett Black's 5 second silent track to the end of the playlist, and these playlists magically start working with Siri again.
https://itunes.apple.com/us/album/silent-tracks-of-various-useful-lengths/366737 722
Give it a shot.