It appears Amazon Drive (and Amazon Photos) no longer renders CR2 images. Customer service insists Amazon Drive doesn't support rendering of CR2 images, but this can't be right.
All CR2 images uploaded on or before October 30th, 2017 render fine (and still show the preview images), however the next time I made an upload (December 26th, 2017 and beyond), they no longer render.
NEF files still render fine.
Notice files Oct 30 and before are ok. Downloaded old 'working preview' images and reuploading them causes the same problem, no previews rendered. checksums of new uploaded files when I download them again are ok. It seems to be a problem with the rendering pipeline.
Have tested against multiple Drive accounts as well and it's not an issue specific to an account.