When a member is filling out mandatory address information during signup, the Postal Code field may not immediately display as mandatory, even if your configurations make this required information.
If the address fields are required (as per configurations under Settings > Apps > People), and the chosen country is either Canada, United States, United Kingdom, or Australia, then the Postal Code field will become mandatory. This is because certain other countries do not have postal codes.
Postal Code not required because no country selected:
Postal Code required once Canada selected: