Wallpaper not saved between reboots
I'm running Catalina 10.15.5 on a MacBookPro. There are 4 Spaces setup each setup to change the wallpaper every hour in random order from a sub folder of my Pictures folder. The Pictures folder and sub-folder is on the MacBookPro's main drive partition. Everything works OK until the MacBookPro is rebooted. After a reboot the wallpaper for all 4 spaces has reverted back to the Catalina defaults.
This used to work in previous versions of the operating system. What can I do to make it work in Catalina 10.15.5? (at the time of writing this is the most recent version of macOS)
Thank you for your help.
MacBook Pro 13″, macOS 10.15