Problems with pixelated photos in photos library after Catalina upgrade

I have about 70-80 photos that were scanned in years ago and had been transferred to my MacBook Pro, mid-2012, from another Mac, again, years ago. These photos looked good until recently, but now are displaying terribly blurry/pixelated. I don't use iCloud for photo storage, so the only thing I can think of is that this difference is somehow since upgrading to Catalina, sometime in the past year??? I was told to upgrade to Big Sur but my computer is not listed as compatible. Note: other, more recently scanned photos seem to display fine. Is this a problem with the Photos app and can it be fixed?