When you go to update the description of your playlist, does it show the previous update, or does it show the original text you set it up with?
It depends. On most of them it reverts to the description as it was originally set (in October). On a couple, some combination of things I was doing had it stick the update afterwards (in November and earlier in December). But on all of them, at the moment, it always eventually reverts to it's last "stuck" description.
Does this happen with new playlist you create as well or just previously existing playlist?
These are all existing playlists with people following them (which I don't want to lose in creating new ones). If all I had to do was create new ones, I wouldn't be having this issue.
Also, see if this happens if you update your playlist from safe mode or a new user account. This can isolate the cause of this issue and possibly resolve:
Well, I suppose I could boot from safe mode and see if it happens again. If I create new user account, I'd have to login as myself to Apple Music as well. But I could try that when I get a chance.
What I may attempt, which may be easier, is seeing what happens when I try to update it from my account on my laptop (as opposed to my desktop, where I usually manage things).