Does adamantine armor protect you from being beheaded by a Dullahan?.

The Dullahan’s battle axe attack states:
“If the dullahan scores a critical hit against a creature, the target must succeed on a DC 15 Constitution saving throw or the dullahan cuts off the target’s head.”

The item description for adamantine armor states:

While you're wearing it, any critical hit against you becomes a normal hit.

A roll of a natural 20 is also a critical hit, which adamantine armor reduces into a regular hit. So, if a creature wearing adamantine armor is hit by such an attack from a Dullahan, does the adamantine armor also prevent the beheading property of the Dullahan?

Quote from Cage2099 >>

A roll of a natural 20 is also a critical hit, which adamantine armor reduces into a regular hit. So, if a creature wearing adamantine armor is hit by such an attack from a Dullahan, does the adamantine armor also prevent the beheading property of the Dullahan?

RAW... looks like it? Or rather, it prevents the beading property from being applied.

I would say it stops it, as the critical hit becomes a normal hit, and normal hits don't trigger the effect.