We cannot enforce uniqueness constraint even if we get success message when alternate key for an entity with existing data is configured. As far as my knowledge goes, this happens because constraint cannot be enforced when existing/deleted records trigger uniqueness constraint violation. We strongly believe that the fact we can configure alternate key for an entity that uniqueness constraint cannot be set for some reason, was never a good idea. It should be fixed ASAP so that alternate key (uniqueness constraint) cannot be configured by users.
Note : Error message indicating uniqueness constraint violation on PowerApps does not pop up when uniqueness constraint is set. We would like to see a bit more user-friendly message whether uniqueness constraint violated, like adding to ErrorKind