I just saw this edit to a very long post. The markdown diff of the edit looks fine, showing that six or so lines were changed, but the "rendered output" of the edit shows everything (even the image) being crossed out and then everything is highlighted as an addition. In the revision history, this is true for every revision of this post starting with #190 for both "inline" and "side-by-side" views.
What I think happened here is that things timed out. But I think it should be able to do better, since so much of the post was unchanged and the markdown view was able to do it right.