QUESTIONS:

WERE YOU TOLD THAT YOUR CHIARI WAS CONGENITAL?

DID THEY TELL YOU WHAT THEY WERE BASING THAT DECISION ON?

https://chiaribridges.org/whats-in-a-name-chiari-malformation/

Because if it was textbook Chiari Information, and they measured your “herniation of the cerebellar tonsils,” and not your “foramen magnum,” that seems to be where the issues start! Check your first radiology report from when you were diagnosed. If you don’t have that report, you have every right to request a copy.

If you have any of these etiological/pathological cofactors, you really might not be congenital:
https://chiaribridges.org/overview-chiari-comorbidities-etiological-pathological-cofactors/