Ah, I can almost guarantee what this is!
Remember the old bug that had summoned troops come in with whatever status effects were on whatever summoned them, in the case of on-death troops?
Infernal King and TDS often revived with status effects that were on them prior, for example.
Gorgotha was the weakest troop. It saw this, and Creeping Death, on cast, saw spots one and three as the weakest troops. It tried to apply a death mark to Gorgotha, in slot one. Had it been any other troop, you would have only placed one death mark, as CD would try to apply DM on an empty spot. But the spot was not empty, it had a thing in it, so CD applied DM to slots one and three, as it should.
But slot one was not empty. So it applied to whatever was there instead.
Suggested fix: Recalculate the second part of a spell’s effect after the first resolves.
Far easier said than done. But, now we know what it is.