[Edi]

Must be. Prophetizing it changed it from a normal Eater of the Dead to type "prophet" even though the monster number is the same. It lists as dead because it went independent (much like charmed and enslaved units are counted dead). Then the second one went indie but is a regular Unfettered, so it stays too. The third one is fine as Eater, but once it reaches Unfettered status, Dominions does a check that says it shoudln't so it goes poof instead.

That's my guess as to how the game logic works with it. Just speculation though, but it fits the observed behavior.