Green dots on HEIC images in Mojave

I have many HEIC pictures, imported from iPhone 7, which show a strange behaviour on my freshly reinstalled iMac (late 2013, Mojave 10.14.1)

If I use Quick View on them they show five rows of green aligned squares on one side.

I I open them in preview the dots disappear.

The real annoying problem is that if I import these pictures in iMovie the dots reappear, so I cannot use them for my movies.

I tested the same pictures on another Mojave iMac and they are ok

I also tested the pictures on a third High Sierra iMac, ok again

This does not happen with JPEG images

I'm lost...