A few years back the OIG stated that you should assume all payers follow Medicare on this as they are the "Gold Standard". If you find a payer that states they do not followMedicare or says "just bill under the provider". It is your responsibility to obtain this policy in writing making sure it is clear in this policy that the provider has not evaluated the patient for a this specific problem and the provider may not be in the office at the time of the encounter, and they are still instructing you to bill using the MD's NPI.
Until you have this as a written policy, should assume that the payer does follow Medicare. If the payer does not wish to credential your midlevel then that midlevel will be restricted from seeing new patients, new problems, or patients when the provider is not in the office who are covered by that payer.