Don't know if this is a bug or a technicality. I was fighting some kobolds and used melt armor which was then blocked by a Mail card. I would have thought that the mail would have been dissolved. Is this a bug or just a technicality since the card is described as "chain scraps" instead of "armor"? It is still a card that provides an armor bonus so I think it should still be affected.
Hard for us to verify that there was a mistake if we don't have a log to look at. Is it possible that the kobold had 1 piece of armor revealed, then you played Melt Armor, which immediately discarded that piece of armor, before running into a previously hidden armor card?