EDIT: I added some (faked) audiogram data to the Health app just to see how that appeared. It also does not show up on that initial screen, you still have to select the "Custom Audio Setup" button and then you can select an audiogram stored in Health. This is not a change new to 18.1, though again maybe prior to 18 it appeared on that initial screen.
Just a thought. Those, AirPods 4, and of course Pro 2 are the only AirPods with rev 7 firmware. All other AirPods are on rev 6.
Both my AirPods (1st gen Pro and Lightning Max), which no longer have audiogram options in Custom Audio Setup since iOS 18.1, are all rev 6.