Guid not generating in Field Maps?

[AaronSchuck1]

I have a feature class related to a table through the TRT_GUID field.  The arcade expression below is entered into the field form, but the guid will not populate.

It's odd because the expression returns a field when tested, but nothing happens within field maps.

var ftrId = $feature.TRT_GUID;
if (IsEmpty(ftrId) == True || ftrId == '{00000000-0000-0000-0000-000000000000}'){
  ftrId = Guid();
  return ftrId
}