How to fix the indexing of photographs on memories

I think there is a problem with the indexing of photographs on my iPhone 12 on IOS 14.3, though this problem has been happening for at least the last couple of updates.

Basically I go to Photos, then choose an album from People and Places.

I edit the default slideshow and I am told I must Add to Memories - all this is fine and has been the same for a long time.

Then I go to Photos & Videos to add and remove photos and when I'm set I select Done.

However what I now find is that some of the photographs I selected have been replaced with some random photographs, so the video is not what I wanted.

This happens more or less every time now.

It never used to happen until one of the IOS updates was done.

Has anybody else seen this and know of the fix - It's very annoying.

I have nearly 23,000 photos on the device and 2,500 videos. Could this be causing a problem?

Any help greatly appreciated

Thanks