iPhone 13 running IOS 15.5 - lock screen image replaced by dark grey screen

Hi All - hope someone can help me with this. I have separate pictures chosen for my iPhone Lock Screen and the Home Screen background. They have always worked normally.

However, whilst my Home Screen background is normal, suddenly in the last week or so the Lock Screen image has been replaced by a plain dark grey background. I have tried playing around with the normal settings, but to no avail.

I then tried to solve it by clicking on a photo in my Photos library and selecting it as a new Lock Screen background.....then when I put the phone to sleep and then touched the screen, the same - a dark grey plain background. The weird thing is that I can see 'underneath' the dark grey a very faint, blurred image of the picture I just chose. I don't understand what has gone wrong.

Can anyone explain and assist me with this issue?

Many thanks in advance.