A quick update on this situation that may help others in the same boat. I talked to Apple Support about my problem and, in the end, they recommended erasing and restoring my Iphone 12. Which, after thinking about it, seemed rather extreme--why would a dramatic reset of my phone clear up the issue of siri not shuffling one of my eight playlists? Also, what would the reset of my phone do for my Homepod--which also won't shuffle that single playlist. Notice that I said "shuffle"--siri won't shuffle that playlist but she will "play" it. Go figure.
All in all, this wasn't a huge deal for me because I could shuffle the playlist manually if I wanted to from my phone to the homepod. But it was a mystery. So this morning I asked siri to shuffle the playlist, but I used different words. Instead of saying "Hey siri shuffle my rock playlist" I said "Hey siri shuffle the rock playlist from my library"--and it worked! This will get me by until the folks at Apple correct the situation. Hope this helps!