There is no contradiction. After the death of an Arahant, there is no mind, no hadaya vatthu.
The mind (and hadaya vatthu) are associated with “this world of 31 realms.”
Nibbana does not belong to this world of 31 realms. As long as existence continues in this world, suffering does not end.
I have explained that in many posts:
“Nibbāna“