Storage on iPhone shows full and prevents most normal functions but massive amount of on-phone storage is unused.

I have visited Genius Bar twice and in both cases they wiped the phone, restored it, stripped down to factory settings but nothing works. They say they don’t understand why it happens. I am on 17.2 update. I note that multiple users have the identical, unsolved problem.

the issue makes the phone unusable for all but the most basic functions.

any ideas?