I noticed that a Helm plugin cannot be upgraded, based on version. The only solution I found was to remove the plugin, prior re-installing it with the new version:

- name: Remove plugin
  kubernetes.core.helm_plugin:
    plugin_name: diff
    state: absent

- name: Install plugin
  kubernetes.core.helm_plugin:
    plugin_path: https://github.com/databus23/helm-diff
    plugin_version: v3.9.8

Is there a better way to do this?