AllocationFailed event is generated when an IP address is requested from an ip address pool using annotation in service that does not have any IP address available for assignment. 

Exact message string " Failed to allocate IP for "e2e-test-networking-metallb-7fqhp/hello-world-43156-1": ["<nil>"] is not allowed in config for service" for AllocationFailed

In past the error message used to indicate no IP available in the pool. This has changed, as caught by automation.
The change is fine as no IP available in the pool message is still provided for AllocationFailed event in different scenario.
The issue is the message string needs more clarity. 

    4.20

  Always  

    1. After installing metallb operator, create metallb CR and l2advertisement CR.
    2. Create IP address pool with just one IP address.
apiVersion: metallb.io/v1beta1
kind: IPAddressPool
metadata:
  name: ip-addresspool-l2
  labels:
    zone: east
  namespace: metallb-system
spec:
  addresses:
    - 192.168.111.65-192.168.111.65
  autoAssign: true
  avoidBuggyIPs: true

     3. Create LB service to see it assigned the IP address from the pool.
4. Create another service but with annotation requesting IP address from pool that does not have any address available to see the issue 
- apiVersion: v1
  kind: Service
  metadata:
    name: hello-idle-1
    annotations:
      metallb.io/address-pool: ip-addresspool-l2
  spec:
    ports:
    - port: 8000
      targetPort: 8080
      protocol: TCP
    selector:
      name: hello-idle-1
    type: LoadBalancer
    allocateLoadBalancerNodePorts: false 

    AllocationFailed event with message "<nil>"] is not allowed in config for service"

  Better error message on why the IP address  allocation failed.  

  oc -n metallb-system get csv
NAME                                    DISPLAY            VERSION               REPLACES   PHASE
metallb-operator.v4.20.0-202508121146   MetalLB Operator   4.20.0-202508121146              Succeeded

  
oc version
Client Version: 4.15.9
Kustomize Version: v5.0.4-0.20230601165947-6ce0bf390ce3
Server Version: 4.20.0-0.nightly-2025-09-01-101753
Kubernetes Version: v1.33.3