Using an example to explain:
1) Admin specify host1 has label=x
2) node=host1:123 registered
3) Get node-to-label mapping, return host1/host1:123
4) node=host1:123 unregistered
5) Get node-to-label mapping, still returns host1:123
Probably we should remove host1:123 when it becomes deactivated and no directly label assigned to it (directly assign means admin specify host1:123 has x instead of host1 has x).