Seems that Infected Corpses are still being summoned with Plague Bearer status for 6 turns, and after it runs out they can no longer apply the debuff. Plague Bearer is cool looking on Infected Corpses for the thematically appropriate cloud of noxious particles and the visual effect should stay, but I don't think Zombie Plague itself is only supposed to function for a brief period of time. I understand it was changed to require you kill the targeted unit within 6 turns, but the Infected Corpses themselves also have a limited time frame of being infectious.
