I agree.  The resulting IPSet from get_ip_rule_set with subnet in the test is IPSet(['0.0.0.0/31', '0.0.0.255/32']).

SUBNET {'next_auto_assign_ip': 0, 'network': {'ip_policy': None}, 'last_ip': 255, 'ip_policy': None, 'first_ip': 0, 'ip_version': 4, 'cidr': '0.0.0.0/24', 'id': 1}

Taking a look also.