Some of thumbnail images in Apple Photos appear to have blocky pink squares when viewing them in “Years” & “Collections” but these strange artifacts disappear entirely when I change the view in Apple Photos to “Moments”. The small pink squares occur only in the thumbnails for some years and not all. I noticed this after I upgraded OSX to 10.11.6 from 10.11.5, and I wonder if that could be the cause as the update scrambled the order of my albums. Before noticing the strange images, I had been working in Lightroom CC 2016 and I wonder whether the alternations that I made to one of my photos while in LR (I moved it back into Photos after editing in LR) could have caused the strange effect. Anyone have any clue?

I am using a new “late 2015” 4.0 gigahz i7 iMac with 5K 27-inch monitor.