This is new for me. It was reported previously on the iOS forum but has not been reported as an Android issue, and was not happening to me until very recently.
Last night, as I do most Monday nights, I created my new playlist of new songs for the week, which had 77 songs on it. Set it to download onto my Android before going to bed, deleting the previous week's playlist first.
When I woke up several hours later it was stuck on 61%. It was still stuck there 45 minutes later, stuck on the same song.
I have found a workaround. ((Create a duplicate playlist, delete the stuck songs when they happen then go back to the original one, and the stuck songs are all there).
However this is going to be very annoying as I am not around all night on a Monday night when I set in motion the playlist download.
I cleared my entire cache in case that fixed it but the issue got worse, and when I started re-downloading some of my playlist, they were regularly getting stuck on songs.
The fact that song appears to be downloaded when I go back to the original list suggests it was completed all along.