Comment 14 for bug 973442

It shouldn't be a huge issue if some ips are not released immediately. If you have this fix included they should be timed out after a while:

https://review.openstack.org/9030/

Also, these fixes should help minimize race conditions on deallocation, so they might fix the issue that you are seeing:

https://review.openstack.org/9041
https://review.openstack.org/10387