Lockscreen Windows Spotlight picture keeps disappearing

Despite ensuring under Personalisation>Lockscreen Windows Spotlight is shown, it works for a couple of days before reverting to the standard blue picture. Resetting only works sometimes.

As a test try setting the lock screen to a picture on your computer. See if it reverts to the blue picture.
It would be helpful if we knew the Windows version and build you are running.
You can find it by opening a command prompt and typing winver.

Apologies, version is 22621.2070
Changing lockscreen to picture brings up the picture I have chosen.

Then try changing it back to spotlight to see if the setting holds or if it reverts back to the blue screen instead.
Problems with Spotlight have various causes. See this article for what can cause it and resolutions.

Thanks for your suggestions, however I have been busy and I just left the setting on Spotlight and after a couple of days it reactivated itself. Have not a clue why. Anyway thanks for your trouble.