Wallpaper not changeable in Mac OS Sequoia 15.4

I've just updated to Mac OS Sequoia 15.4 on both my M3 iMac and my M3 MacBook Air. On both, I had been using a solid custom wallpaper colour. Now my wallpaper on both computers is white. I can change to a picture, I can choose one of the few provided colours – but if I try to change to my custom colour, or any custom colour, the wallpaper reverts to white. This seems like a bug, and it's the first time in many years of Mac updates it's happened to me. How can I fix this?