Prometheus Cluster manifest (prometheus-cluster.yaml) in K8s 1.18.3

Due to customer requirement, I am testing Portworx Enterprise 2.5 in Kubernetes 1.18.3. on-premise (pure Kubernetes)

I have an interesting find: When deploying Prometheus, using prometheus-cluster.yaml according to documentation gives an error - namespaceSelector is considered invalid argument.
If I just ignored lines regarding namespaceSelector deploy goes OK and works fine, including interacting with Grafana. I understand there are situations Prometheus has it own namespace (like in AWS EKS) but I just would like to mention this situation so if anybody comes with this problem, it makes no difference if you deploy prometheus in kube-system (in my history, no customer requested to be in any other namespace in case of on-premise kubernetes)
Regards and any comments would be welcome