This recently started happening to me. I'll do an import into my catalog. The images will successfully load. When I look at the images in the browser, a few of them will be blurry, b&w thumbnails of older images that I've imported. But, when I select the image in the browser, the correct image is displayed in the viewer.
Has anyone else noticed this?
I'm on an iMac using Mojave 10.14 and CO 11.3.