The spec for @font-palette-values states this about relative units, calc(), var() and env():

Math functions, such as calc(), and also var(), and env(), are valid within descriptor values in a @font-palette-values rule. They are evaluated within the context of the root element. Relative units are also evaluated within the context of the root element.

but, it doesn't state when that evaluation should take place.

The spec should make it clear when they values are evaluated and if changes to the root element should cause them to be re-evaluated.