Photos in Finder

The sequoia update seems to have compromised photo handling on my Macbook Air.

When photos are downloaded to Downloads, all of the thumbnails are visible.

However, if they are then copied into a folder in (for example) Desktop, most of the thumbnails disappear.

The photos are still there - double clicking opens them immediately, and the get info button shows the appropriate size, but the thumbnails are not visible.

Not an issue before the sequoia update, but a big problem now.

Does anyone know how to resolve this?