Port creation fails due to "IP address XX.XX.XX.XX already allocated in subnet" even though there is no port which has the IP address

Issue

• Port creation with a fixed-IP fails due to IP address XX.XX.XX.XX already allocated in subnet error

  (undercloud) [stack@undercloud ~]$ openstack port create --network external --fixed-ip subnet=aaaaaaaa-aaaa-aaaa-aaaa-aaaaaaaaaaaa,ip-address=192.168.0.1 port1
  ConflictException: 409: Client Error for url: http://10.0.0.1:9696/v2.0/ports, IP address 192.168.0.1 already allocated in subnet aaaaaaaa-aaaa-aaaa-aaaa-aaaaaaaaaaaa
  

• However, there are no ports which have the IP address

  (undercloud) [stack@undercloud ~]$ openstack port list |grep 192.168.0.1 
  
    ==> There are no ports which have 192.168.0.1 
  

• This issue also can occur on openstack overcloud node provision command.

  (undercloud) [stack@undercloud ~]$ openstack overcloud node provision ...
      :
  | aaaaaaaa-aaaa-aaaa-aaaa-aaaaaaaaaaaa |      FATAL | Provision instance network ports | localhost | error={"changed": true, "error": "ConflictException: 409: Client Error for url: http://10.0.0.1:9696/v2.0/ports, IP address 192.168.0.1 already allocated in subnet aaaaaaaa-aaaa-aaaa-aaaa-aaaaaaaaaaaa", "msg": "Error managing network ports ConflictException: 409: Client Error for url: http://10.0.0.1:9696/v2.0/ports, IP address 192.168.0.1 already allocated in subnet aaaaaaaa-aaaa-aaaa-aaaa-aaaaaaaaaaaa", "node_port_map": {}, "success": false}
      :