Hello Brett,
The key point is with this part of your comment: “Nibbana is pure mind beyond matter.”
Upon attaining Arahanthood, that Arahant’s mind becomes pure. That is called the “saupadisesa Nibbana” or “Nibbana with residue.” That is because the Arahant still lives with that physical body and hadaya vatthu.
– However, at the moment of the death of the physical body, the “mental body” (gandhabba) with the hadaya vatthu comes out of that dead body and dies too. Normally, when a hadaya vatthu associated with a given bhava (existence) dies, a new hadaya vatthu is created by kammic energy, if there is any anusaya left.
– Since the Arahant has no “defiled gati” or anusaya left, a new hadaya vatthu will not be created in this case. Therefore, that lifestream ends, and that is Nibbana! It is called “anupadisesa Nibbaba” or “Nibbaba without any residue.”
The key point here is that hadaya vatthu is not eternal. When a lifestream switches from one existence (say human existence) to another existence (say, Deva or animal), a new hadaya vatthu matching that new existence will arise due to corresponding kammic energy.