Thanks for this.
Almost certainly a mixture of iPhone SE, Canon IXUS and Canon 5D MkII. iPhone via iCloud and or USB, IXUS via built in MacBook Pro card reader, 5D MkII via USB lead. I'll review individual photos and see if it is predominantly one source. I have 25,000 photos in the database. Also some will have been an import from my Synology NAS database as I have migrated from using Lightroom/Photoshop to Photos/Affinity.
I have been using photos for a number of years and it is only in recent versions of Catalina that this has become apparent. I'm pretty sure it's not the case that it has been a problem since first upgrading to Catalina. As I say within the Photos library and individually they dis[play correctly. Perhaps it's related to the size of the image?
Will come back to you once I've reviewed them.
Cheers,