It didn’t correctly work that way in the first place. Previously, reports were saying that putting Zerek’s Deathrattle on another minion (with a spell cast on it) would summon another Zerek, even though the deathrattle’s text didn’t name him specifically. This isn’t the first report that Zerek’s Deathrattle now no longer works at all outside the minion it originated on - Undatakah + Zerek bug.
What other Deathrattles did Undatakah have? Test Subject? Or any of the ones that return a dead minion to your hand or deck?