Hello Elaine!
Actually, what ResourceMap does is to autogenerate the luhn value ONLY if there is not a value in the POST request. If a value is provided, then it uses it (if it is valid -luhn check, format and uniqueness in the collection-).
So, you should be able to have the luhn field in both collection: In the curation tool, since the value does not exists yet, ResourceMap will generate a new one; and in the public one, since you create the facility with an existing luhn value - the matching one from the curation collection- ResourceMap will keep this value.
Best,
Caro