Please, answer some short questions which should help us to understand your problem / question better?

• Which image of the operator are you using? e.g. ghcr.io/zalando/postgres-operator:v1.13.0
  ghcr.io/zalando/postgres-operator:v1.12.2

• Where do you run it - cloud or metal? Kubernetes or OpenShift? [AWS K8s | GCP ... | Bare Metal K8s]
  bare metal

• Are you running Postgres Operator in production? [yes | no]
  yes

• Type of issue? [Bug report, question, feature request, etc.]
  feature request

Some general remarks when posting a bug report:

• Please, check the operator, pod (Patroni) and postgresql logs first. When copy-pasting many log lines please do it in a separate GitHub gist together with your Postgres CRD and configuration manifest.
• If you feel this issue might be more related to the Spilo docker image or Patroni, consider opening issues in the respective repos.

If the <namespace>.<username> attribute is set in the manifest, it should create <username> still instead of a user that is literally <namespace>.<username>. Either that or allow the option to only create the <username> user only.