In the specific case of normalization, I think most IDNA implementations
will rely on a normalization implementation created elsewhere - few
IDNA implementations will come with their own normalization routines.
This is because normalization is difficult to implement, so you rather
reuse than reinvent. Then, when the underlying normalization routine
is changed, the IDNA implementation also changes - most likely without
the author of the IDNA implementation (let alone its user) even knowing.
In that scenario, the implementor of IDNA has little control over what
specific incarnation of normalization is used.