https://www.phpbb.com/community/viewtopic.php?p=13837776#p13837776 suggests that the Newly Registered Users group, by default, has global 'Can change profile field information' set to Never. However, when upgrading from phpBB 3.0.12 the php_acl_roles_data table does not have an entry with auth_option_id = u_chgprofileinfo WHERE role_id = ROLE_USER_NEW_MEMBER and thus the default becomes No.