The PAM realm is per-node, not cluster-wide. The behavior you suggest could be implemented of course, but it would be a breaking change, because some people might already rely on the current behavior. I'd rather document the fact more explicitly, it's just the implicit "As Linux PAM corresponds to host system users" right now: https://pve.proxmox.com/pve-docs/chapter-pveum.html#user-realms-pam
You're right, it's just somehow irritating because you are configuring the users/passwords in the Cluster-View, not in the Node-View.
To add an Additional Section in the Node View for PAM Users is not the right way i think either, but then it probably should be done or alternatively PAM-Users should be updated on both nodes if in Cluster-View.