Stripe prices created on a one-time basis directly on a Subscription during an update means the Price was never active, to begin with, which is why it can't be unarchived or fetched. Auto-Archived prices are not be returned through a List call via v1/prices, however, you can retrieve these prices via the API by making a request to retrieve the price though Get v1/price/<id>. Apparently, this is the only way for the information to be returned via the API.

Ask: please update the Fivetran Stripe connector to get all prices via the prices api, and by checking for active subscriptions that use these archived prices. Thanks