I only recently started trying the playlist feature for music and at first all was well. The thumbnail images appeared to be generated from four randomly selected album covers included in the playlist.
However, recently the playlist thumbnails have been changing into things like the attached image. I don’t actually look often enough to know whether these changes coincide with server version updates, but I do know that there have been at least a few updates since I created my first playlist (I only have four so far and three like like the image while one is pristine and perfect.)

My Plex server is Version 1.9.7.4460 and is running under Windows 10 Pro, Build 1709.
I forgot to ask my question!
How can I correct the playlist thumbnails. FWIW, I tried deleting the smallest list and recreating it from scratch and the thumbnail looked just as messed up.
I have noticed that the images will change (for better or worse over time). It looks like a more robust image generation algorithm is needed by the Plex Devs.
FWIW, this is still happening (for me, at least) with Plex Media Server Version 1.12.0.4829.
Bump. Still an issue for me a year later. PMS is the latest plexpass version.