CA Ins Code §796.04

A health insurer that provides coverage for hospital, medical, or surgical expenses that authorizes a specific type of treatment for services covered under a policyholder’s contract or plan by a provider shall not rescind or modify this authorization after the provider renders the health care service in good faith and pursuant to the authorization for any reason, including, but not limited to, the insurer’s subsequent rescission, cancellation, or modification of the insured’s or policyholder’s contract or the insurer’s subsequent determination that it did not make an accurate determination of the insured’s eligibility.