The Master, Transcendent's Mutant Creatures

Just did a game where I had The Master, Transcendent, Thassa, Deep-Dwelling on the battlefield and milled Ulamog, the Infinite Gyre.

Ulamog, the Infinite Gyre triggers the moment it enters the graveyard and holding priority I used The Master's activated ability, bringing Ulamog as a 3/3 green mutant to the battlefield.

At the end of the turn I used the triggered ability of Thassa, Deep-Dwelling targeting Ulamog. The trigger resolves but Ulamog stays a 3/3 green Mutant.

According to the text and rules:

The Master, Transcendent ability:
"Tap: Put target creature card in a graveyard that was milled this turn onto the battlefield under your control. It's a green Mutant with base power and toughness 3/3."

That means that the creature is not a copy but the original "transformed" as a 3/3 green Mutant. It should stay like this as long as it stays on the battlefield because it entered the battlefield through Master's ability.

Thassa, Deep-Dwelling ability:
"At the beginning of your end step, exile up to one other target creature you control, then return that card to the battlefield under your control."

Rule:
"400.7. An object that moves from one zone to another becomes a new object with no memory of, or relation to, its previous existence."

So when Ulamog, the Infinite Gyre reenters the battlefield under the exile/return effect of Thassa, I believe it should return as an original 10/10 and not a 3/3.

I don't know if I'm missing something but I checked the log and the Thassa's ability indeed triggered and resolved.

No source file or test is named. Start by reproducing the interaction in an XMage game and inspect the game log around The Master's ability, Thassa's exile-and-return trigger, and Ulamog's zone changes. Done means the implementation matches the stated Magic rules for Ulamog's characteristics after it returns.

Written by the indexing model from the issue text.