ip_count_list increments even if port creation is not successfull

In a setup of contrail 4.1.0 and RHOSP 10, when neutron port creation fails due to an exhausted subnet, the ip_count_list keeps incrementing:

[heat-admin@overcloud-contrailcontroller-0 ~]$ curl -H "X-Auth-Token: `openstack token issue -c id -f value`" http://192.168.122.11:8082/virtual-network/18067a99-ed12-4a74-8b50-92577dc27bdb/subnet-ip-count -H "Content-Type: application/json" -d '{"subnet_list": ["b2ae906c-fb7d-4549-b88d-420e0bdb553e"]}' |python -m json.tool
  % Total % Received % Xferd Average Speed Time Time Time Current
                                 Dload Upload Total Spent Left Speed
100 80 100 23 100 57 142 352 --:--:-- --:--:-- --:--:-- 354
{
    "ip_count_list": [
        25
    ]
}
[heat-admin@overcloud-contrailcontroller-0 ~]$ neutron port-create --name testsrv01 `openstack network show sriov-test -c id -f value`
Bad port request: Virtual-Network(['default-domain', 'admin', 'sriov-test']) has exhausted subnet([]).
Neutron server returns request_ids: ['req-760c87c1-05fb-422a-a81c-1311e7b749d0']
[heat-admin@overcloud-contrailcontroller-0 ~]$ curl -H "X-Auth-Token: `openstack token issue -c id -f value`" http://192.168.122.11:8082/virtual-network/18067a99-ed12-4a74-8b50-92577dc27bdb/subnet-ip-count -H "Content-Type: application/json" -d '{"subnet_list": ["b2ae906c-fb7d-4549-b88d-420e0bdb553e"]}' |python -m json.tool
  % Total % Received % Xferd Average Speed Time Time Time Current
                                 Dload Upload Total Spent Left Speed
100 80 100 23 100 57 148 369 --:--:-- --:--:-- --:--:-- 370
{
    "ip_count_list": [
        28
    ]
}

[heat-admin@overcloud-contrailcontroller-0 ~]$ neutron net-show 18067a99-ed12-4a74-8b50-92577dc27bdb
+-----------------------+--------------------------------------------------------------------------------------------------------------+
| Field | Value |
+-----------------------+--------------------------------------------------------------------------------------------------------------+
| admin_state_up | True |
| created_at | 2018-05-19T09:09:37.819442 |
| fq_name | default-domain |
| | admin |
| | sriov-test |
| id | 18067a99-ed12-4a74-8b50-92577dc27bdb |
| name | sriov-test |
| port_security_enabled | True |
| project_id | db666c41aefc4a4581655e3c2e02627c |
| router:external | False |
| shared | False |
| status | ACTIVE |
| subnet_ipam | {"subnet_cidr": "8.8.8.0/24", "ipam_fq_name": ["default-domain", "default-project", "default-network-ipam"]} |
| subnets | b2ae906c-fb7d-4549-b88d-420e0bdb553e |
| tenant_id | db666c41aefc4a4581655e3c2e02627c |
| updated_at | 2018-05-19T09:51:16.185235 |
+-----------------------+--------------------------------------------------------------------------------------------------------------+
[heat-admin@overcloud-contrailcontroller-0 ~]$ neutron subnet-show b2ae906c-fb7d-4549-b88d-420e0bdb553e
+-------------------+-----------------------------------------+
| Field | Value |
+-------------------+-----------------------------------------+
| allocation_pools | {"start": "8.8.8.2", "end": "8.8.8.10"} |
| cidr | 8.8.8.0/24 |
| created_at | |
| dns_nameservers | 0.0.0.0 |
| enable_dhcp | False |
| gateway_ip | 8.8.8.1 |
| host_routes | |
| id | b2ae906c-fb7d-4549-b88d-420e0bdb553e |
| ip_version | 4 |
| ipv6_address_mode | |
| ipv6_ra_mode | |
| name | b2ae906c-fb7d-4549-b88d-420e0bdb553e |
| network_id | 18067a99-ed12-4a74-8b50-92577dc27bdb |
| tenant_id | db666c41aefc4a4581655e3c2e02627c |
| updated_at | |
+-------------------+-----------------------------------------+