Dr. Stanford said she was not challenged for the rest of the two-hour flight. As they were descending, she said, one attendant told her they would not need to check her license anymore because “it seems like you were able to handle everything.” She interpreted the encounter as biased because of the persistent questioning about whether she was a physician even after she had provided proof. “It never stopped,” she said. “I just couldn’t figure out why we were having this discussion.” Delta apologized to Dr. Stanford on Wednesday and said it was investigating, according to the email it sent her. On Thursday, Anthony Black, a Delta spokesman, noted that the airline had changed its policy about medical credentials in 2016 to say flight attendants are not required to verify credentials from someone who says she or he is a physician, physician assistant, nurse, paramedic or emergency technician.