It happens in Y (again, you don’t see his dead body but there’s no reason to assume he isn’t dead…does the classic villain “my plans are foiled so I’ll just blow up my whole damn lair while I’m still inside it”). However, my understanding is that X version has its plot emphasis on him obtaining eternal life for himself and Team Flare members, so I guess he’s just like…alive under all the rubble or something, IDK.