It does receive the MQTT command, but nothing happens on the NOTI:

0|mm     | [MQTT bridge] MQTT message received. Topic: bathroom/mirror, message: MONITOROFF
0|mm     | [2020-10-16 09:23:53.728] [LOG]
0|mm     | [MQTT bridge] MQTT -> NOTI issued: 'REMOTE_ACTION', payload: {action: 'MONITOROFF' }

Same thing happens for MONITORON, DIM and UNDIM.

The mqttdictionary file looks like this now:

var mqttHook = [
    {
      mqttTopic: "bathroom/mirror",
      mqttPayload: [
        {
          payloadValue: "MONITOROFF",
          mqttNotiCmd: ["MONITOROFF"]
        },
        {
          payloadValue: "MONITORON",
          mqttNotiCmd: ["MONITORON"]
        },
        {
          payloadValue: "UNDIM",
          mqttNotiCmd: ["UNDIM"]
        },
        {
          payloadValue: "DIM",
          mqttNotiCmd: ["DIM"]
        },
      ],
    },
  ];
var mqttNotiCommands = [
    {
      commandId: "MONITOROFF",
      notiID: "'REMOTE_ACTION'",
      notiPayload: "{action: 'MONITOROFF' }"
    },
    {
      commandId: "MONITORON",
      notiID: "'REMOTE_ACTION'",
      notiPayload: "{action: 'MONITORON' }"
    },
    {
      commandId: "DIM",
      notiID: "'REMOTE_ACTION'",
      notiPayload: "{action: 'BRIGHTNESS', value: '50')"
    },
    {
      commandId: "UNDIM",
      notiID: "'REMOTE_ACTION'",
      notiPayload: "action: 'BRIGHTNESS', value: '255'}"
    },
  ];

  module.exports = { mqttHook,  mqttNotiCommands};

And the notification looks just like the one here in the documentation example here:

this.sendNotification('REMOTE_ACTION', {action: 'RESTART'});

The ipWhitelisting has been both tried with [(“127.0.0.1”,“localhost”,“my.ip.range.0/24”)] and also with just [].
I can issue the commands MONITOROFF/MONITORON directly in the Remote-Control menu at /remote.html

Any input from the developers on this? @Serge , @Bugsounet, @Jopyth :-)

Is it a MMM-MQTTbridge problem or a MMM-Remote-Control problem?