Distorted photo previews in Apple Photos app

[Armendegga]

See screenshot for details. Can't figure out how to fix... seems to be on random photos. If I open the picture up by double clicking, it shows fine... then when I close it, the preview is back to being distorted.

Any ideas on what could be causing it? This is using the Photos app on a 2013 iMac running latest version of Catalina.

 

[Armendegga]

Bump... Hoping someone might know what is causing this.

 

[Ruggy]

Not specifically I'm afraid.
I think the first thing to try is the photos library repair tool:

How to use the Photos Repair Library tool on your Mac – Apple Support (UK)

If your Photos library won't open or if the Photos app behaves unexpectedly on your Mac, the Photos Repair Library tool may help.

support.apple.com

Your problem concerns 'thumbnails' some of which have become corrupted.
Rebuilding the library should rebuild the thumbnails and cure the problem.
Hopefully this helps.
Best