This is actually the correct behavior and how it should have always been working; there isn't any reason this should have changed in 2.2. This option means that the user can set the value once and then it can't be changed.
Worth noting that this doesn't necessarily mean they can only set it at registration. This is significant if you add a custom field after someone registers or if you make the field non-required (as it won't be considered set until they select a value).