Page previews for some pages show a pixelated icon instead of the correct thumbnail.
First noticed on cswiki in T182321#3990934. I can also reproduce the same on a few enwiki pages. From the Recent Changes page:
Strangely enough, it's not all of them. Here are some from RC that show the correct thumbnail for me:
My best explanation at this time for the ones showing the correct thumbnail is probably caching.