The hidden image is shown in the "Rearrange images" view, but not the "Add or remove images" view. If this is not a bug, it is at least a reasonable expectation for a feature.

FTR, I'm looking to remove the last image from http://imgur.com/a/ETbgx/ and did use the http://imgur.com/removalrequest form with the image URL: http://i.imgur.com/EBhiBh.png