¯\_(ツ)_/¯
It's difficult to provide an explanation regarding the changes in the anime given the anime provides no semblance of an explanation to speculate anything worthwhile, but if I had to theorize... Her immortality may or may not work the way you think it does. Rather than a 'double immortality', one may just reinforce the other. As a Mother of Life, Tiamat is born without an inherent concept of 'Death' and the existence of her Paradox Restoration ability ensures that 'Death' is unable to be imposed onto her rather than work as a form of immortality.
They were forced to drop Tiamat into the Underworld because her Paradox Restoration inherently rejects 'Death' (which King Hassan embodies) from touching her so when they managed to circumvent that ability, King Hassan became able impose on her and remove her immortality. Her Paradox Restoration ability rejects 'Death', which renders her incapable of 'Dying', but rather than a more literal, traditional sense of the word, it could be that the ability simply rejects the concept of 'Death' being imposed onto Tiamat's Saint Graph so under the circumstances that you're able to circumvent her Paradox Restoration and impose 'Death' onto her, which also simultaneously dropped her God-tier Saint Graph to the level of a standard Servant, it could be that Paradox Restoration becomes unable to function, i.e. it doesn't work as an ability that can make a mortal being immortal but rather an ability that ensures an immortal being cannot have the concept of 'Death' imposed on her or it could be that having her Saint Graph dropped to the level of a standard Servant just rendered Paradox Restoration ineffective.
Like I said, it's difficult to explain because this is somewhat hypothetical and the anime puts us in a situation that has no precedent for an explanation, but what I'm trying to say is that Tiamat's Immortality and her Paradox Restoration are abilities that compliment one another: her Immortality makes her... well... immortal... and what Paradox Restoration does is ensure that this Law is upheld so long as Tiamat's role as a Mother is in function, i.e. so that it's impossible to impose a concept of 'Death' onto her under unnatural means.
One ability removes the concept of 'Death' and makes her immortal, while the other rejects 'Death' from being imposed onto her so that the former would be upheld as an absolute - in other words it's a fail-safe. You could counter the former by just imposing 'Death' onto her, but the latter ensures that doesn't happen so it rejects any such attempts. If you shut down the later, you are now able to do that and in the circumstances that the latter re-activates after you do that, it might not be able to return the concept of Immortality she had already lost. Or it could be that it's unable to be re-activated altogether in the circumstances that her Saint Graph is dropped to that of a standard Servant because she loses her role as an Earth Mother Goddess.
TL;DR;
Wanna kill Tiamat? Nope, her Immortality no-sells it
Okay, use Anti-Immortality weapons (Azrael) instead? Nope, her Paradox Restoration no-sells it
But if you lock Paradox Restoration and become able to use Anti-Immortality weapons on her, the Paradox Restoration could end up ineffective |