Unable to verify if this iPhone has genuine apple camera after ios 15 update on 12 pro max with warranty

Take it to Apple Store for repair or replacement under warranty or you can reach out to Apple Support directly. Depending on your region, you may be able to reach them here: contact Apple Support 

You are correct.

I think the advice is if your phone is in warranty then have Apple replace it.

If the phone is out of warranty and the camera still works, then live with the message. Costs £500 to fix it. Fixing it in a third party shop will bake the message in.

Personally I occasionally offer the backup and restore option first on the basis that:

1. it has to be done prior to repair anyway so may as well practice it.
2. There's a tiny chance it may fix it.

There are quite a few separate threads relating to this issue but I have not yet seen any solution other than returning the iPhone for replacement.

e.g.: Software restore to get rid of (wrongful) genuine camera message?