Cannot apply a configmap to nginx-ingress owned by github-managed-apps

I’m running a group-level Kubernetes cluster via Gitlab, which is behind a Gitlab managed nginx ingress controller (and cert manager). I’m trying to apply a configmap to change the default redirect code from 308 to 301 (because OpenGraph doesn’t support 308 redirect).

I’m having trouble finding documentation on how to do this. I can apply a configmap, but that never gets applied.

Has anyone had any success in applying a configmap to nginx in the github-managed-apps namespace?