Hmm, it’s a weird one.
I had a look at this particular Europa Clipper XKCD using an iOS man in the middle proxy as an example
For reference, this story can be found at story_hash 8268588:0922b2
The underlying image itself is fine and so is the Newsblur proxied image found at https://imageproxy.newsblur.com/sc,sfKD24miH-CTMKTf7rW8pHqBYgUjMSlPtg3494kUJXG0=/https://imgs.xkcd.com/comics/europa_clipper.png
That would suggest the actual content is making it to the iOS app just fine. I can also hold on the broken image, hit “Copy image” and successfully paste the image into Apple Notes which would suggest the image is also not being corrupted locally in any way.
Given all that, it seems everything is technically fine, just that rendering fails for particular articles. I can’t spot any obvious patterns. Most XKCD posts all work fine on iOS and they’re all png so it’s nothing weird like one image happening to be a unique format 