Custom plugins are not copied

[amedee]

I have this in my meta/main.yml:

  - role: geerlingguy.munin-node
    munin_node_allowed_ips:
      - '^127\.0\.0\.1$'
      - '^::1$'
      - '^IP_ADDRESS$'
    munin_node_config: {
      "backup_size": {
        "user": "root"
      },
      "mailbox_*": {
        "user": "root",
        "env.mailbox_user": "EMAIL"
      }
    }
    munin_node_install_plugins:
      - src: files/etc/munin/plugins/backup_size
      - src: files/etc/munin/plugins/mailbox_messages
      - src: files/etc/munin/plugins/mailbox_size

My custom plugin scripts are not installed. The config is correct, though.

I am going to assume that there is a simple error at my end, that I got something wrong in the syntax of munin_node_install_plugins. How do I fix this?