kustomize merge in kubernetes is overriding instead

I am trying to use Kustomize to do a Patch Strategic merge on the following yaml.

y1:

apiVersion: iam.cnrm.cloud.google.com/v1beta1
kind: IAMPolicy
metadata:
  name: storage-admin-policy
  namespace: cnrm-system
spec:
  resourceRef:
    apiVersion: iam.cnrm.cloud.google.com/v1beta1
    kind: IAMServiceAccount
    name: storage-admin
    namespace: cnrm-system
  bindings:
    - role: roles/iam.workloadIdentityUser
      members:
        - serviceAccount:mysten-sui.svc.id.goog[monitoring/thanos-compactor]

y2:

apiVersion: iam.cnrm.cloud.google.com/v1beta1
kind: IAMPolicy
metadata:
  name: storage-admin-policy
  namespace: cnrm-system
spec:
  resourceRef:
    apiVersion: iam.cnrm.cloud.google.com/v1beta1
    kind: IAMServiceAccount
    name: storage-admin
    namespace: cnrm-system
  bindings:
    - role: roles/iam.workloadIdentityUser
      members:
        - serviceAccount:mysten-sui.svc.id.goog[monitoring/test-compactor]

I am using the following kustoomization file:

resources:
- y1.yaml

patchesStrategicMerge:
- y2.yaml

My requirement is that, I want the YAML to be clubbed something like this:

apiVersion: iam.cnrm.cloud.google.com/v1beta1
kind: IAMPolicy
metadata:
  name: storage-admin-policy
  namespace: cnrm-system
spec:
  resourceRef:
    apiVersion: iam.cnrm.cloud.google.com/v1beta1
    kind: IAMServiceAccount
    name: storage-admin
    namespace: cnrm-system
  bindings:
    - role: roles/iam.workloadIdentityUser
      members:
        - serviceAccount:mysten-sui.svc.id.goog[monitoring/test-compactor]
    - role: roles/iam.workloadIdentityUser
      members:
        - serviceAccount:mysten-sui.svc.id.goog[monitoring/thanos-compactor]