In SiteConfig when you were scaling-out a SNO adding a node into the SiteConfig it worked as expected. The BMH CR was created on the Hub.

Now with SiteConfig operator via ClusterInstance, this operation cannot be done. 

ACM latest

100%

1. Deploy a SNO cluster via the ClusterInstance CR.
2. Add a node to the ClusterInstance CR.
3. The extra node BMH is not created on the Hub.

$ oc -n sno get clusterinstances.siteconfig.open-cluster-management.io sno-ai -ojson | jq .status.manifestsRendered

[
  {
    "apiGroup": "extensions.hive.openshift.io/v1beta1",
    "kind": "AgentClusterInstall",
    "lastAppliedTime": "2025-05-14T08:48:47Z",
    "message": "admission webhook \"agentclusterinstallvalidators.admission.agentinstall.openshift.io\" denied the request: Attempted to change AgentClusterInstall.Spec which is immutable after install started, except for ClusterMetadata,IgnitionEndpoint fields. Unsupported change: \n\tNetworking.UserManagedNetworking: (true => false)",
    "name": "sno",
    "namespace": "sno",
    "status": "failed",
    "syncWave": 1
  }, 

The SNO cluster is scaled-out

https://redhat-internal.slack.com/archives/CUPJTHQ5P/p1745508459299059