Applying Policy resource after installing Istio 1.6 canary

My cluster was running Istio 1.4 and I installed Istio 1.6 as a canary deployment.

I still want to apply the Policy resource to namespaces using the Istio 1.4 control plane.

This seems to work on several of my clusters but on one particular gcp cluster I am unable to apply because validation does not recognise the Policy resource.

I have confirmed that Istio 1.4 is running on this cluster and there is an existing Policy resource in a namespace from before Istio 1.6 was installed.

Does anyone have insight into why this may occur?