This may not be a case of image rot, as it were. However, it is something people have to be mindful of and see to its fixing whenever they run into it.
When we setup the i.stack address, we subsequently performed a global replace of all imgur URLs that were provided after a specific date. This affected even the ones which weren't uploaded through Stack Exchange, which means they don't actually have a corresponding i.stack image. This is going to be the most likely cause of any broken images with the i.stack address provided.
The proper fix would be to not revert the link back to the non-i.stack, but to re-upload the non-i.stack image through the Stack Exchange UI so that we will have a persistent copy.
See also: a relevant Meta Stack Overflow post on the subject