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Looks like partially loaded progressive JPEG, maybe your viewer ditched everything after the first pass. I don't know if windows programs share a same JPEG library in the system, however if this is the case then nearly all programs could be screwed for your current OS installation.

Have you tried viewing the exact file that show pixelated on other computers or devices?

Edit: I somehow found this theory stupid. If it's you then many other things than AP scans should have broken. If it's some uploader using stupid compression software then most scans should still work for you. And I don't think AP would recompress their images with progressive JPEG themselves.

Anyway just try viewing, preferably re-downloading on other machines. If it's still wrong then you know for sure.