iPhone won't save Favorite contacts

(First noticed in iOS 15.6, but it may have existed earlier. Still present in 15.7.)

I am no longer able to save contacts as Favorites. I go through the same steps as previously worked:

-- select contact from Contacts list

-- tap Add to Favorites

-- tap Call

-- tap the number I want to add as a favorite

But the number is not added to my Favorites list. There are plenty of favorites there already, added some time ago through exactly the same steps.

There are various fixes cited on third-party sites, but none of them apply in my case. I'm not syncing contacts with other accounts or with iCloud; there is no duplication; there are no linked contacts; I'm on the latest iOS.