Hearthpwn

So in a recent arena game i encountered a strange interaction involving The Darkness, Unleash the Hounds, and Spreading Plague. My opponent played The Darkness early on, thus occupying one of his spots with a dormant minion. Unleash the Hounds recognized this, and created a hound for the enemy minions including The Darkness, which was still dormant. However, on the following turn my opponent played Spreading Plague, which did not recognize The Darkness as a minion, and created an extra beetle despite having an equal number of minions with the dormant Darkness included.

Is this a bug or is the wording on the cards different enough that this is the proper function?