Hi we are facing the exact same problem. Did you find any solution in the meantime?
I think I just found out what’s going on. When you do such an error a static ip address is reserved based on the data field of dns record. In your case 10.81.0.1,10.81.0.3.
You can try do the following:
maas root ipaddresses release ip=10.81.0.1
maas root ipaddresses release ip=10.81.0.3
In my case I managed to delete both the reserved ip and also the stale dns record