Hearing Aid Function on AirPods Pro 2 Stopped Working

This week my father's AirPods Pro 2's hearing aid function stopped working. While he tends to "break" things with technology, in this case all of the settings are the same they've always been. When he first got them I checked them out and the volume increase is significant. Now there is no change in volume, at least while watching TV. To him and me (my hearing is fine), the TV sounds the exact same volume with or without the AirPods. Basically, just Transparency Mode.

The volume for the AirPods themselves is at maximum as well as Hearing Assistance volume. They're set for Transparency On (the other settings never seemed to work when using them as a hearing aid). The firmware for the AirPods and case are the latest. iPhone is on iOS 26.0.1. I've reset the phone a couple times. I've turned off BT and turned it back on a few times. I even "forgot" the AirPods and repaired them. And the tips are clean.

The only thing I can think of is to redo the entire hearing test to see if that resets something. Looking for suggestions of things we haven't tried yet. FYI, the AirPods were purchased 367 days ago, so it's not like they're old.