I understand that if a Moldgraf Monstrosity with undying through, maybe Undying Evil, will come back into play instead of being exiled by it's own ability when it dies (assuming you stack the abilities appropriately).
My question is, why? Moldgraf Monstrosity doesn't state "exile it from the graveyard" but rather "exile it". I'm looking for the reasoning here and I must be missing it.
It's because its ability triggers when it hits the graveyard. The game treats the Monstrosity on the battlefield as a new object. The ability tries to exile an object that no longer exists, and is completely independent from the Monstrosity on the battlefield.
A similar situation would be if you exile cards with a Karn and then bounce it back to your hand. If you played the Karn card again, and ended up activating its ultimate ability, the cards that were exiled by the first Karn object won't be brought back by the second's ability.
It's because when its ability triggers when it hits the graveyard. If it isn't there when the ability tries to resolve, in this case because of undying, the game treats it as a new object. The ability tries to exile an object that no longer exists and is completely independent from the Monstrosity on the battlefield.
Ah, of course. It becomes a new object with each transition from one zone to another. It's own effect is exiling "this object" when it triggers and it becomes a new object before it resolves. Thank you very much.