pm2 watch argument doesn't watch the files

I am using PM2 with the following configuration:

module.exports = {
  apps : [{
    name: 'sandbox',
    script: 'index.js',
    args: ["PORT=8084", "--color"],
    instances: 1,
    autorestart: true,
    watch: 'index.js',
    out_file: "logs/out.log",
    node_args: "--trace-warnings"
  }]
};

All works well except that changes in index.js don't trigger restart.

I have tried many things:

• adding the absolute path in script and in the watch
• adding cwd with the absolute path
• Using variations in the watch like ./index.js or ../ or ./ or true
• Removing autorestart

Additional info:
My app use express

The status shows that watch is enabled:

│ status            │ online
│ name              │ sandbox
│ version           │ 1.0.0
│ restarts          │ 0
│ uptime            │ 8m                                                 │
│ script path       │ /var/www/api/index.js                              │
│ script args       │ PORT=8084 --color                                  │
│ error log path    │ /home/ubuntu/.pm2/logs/sandbox-error-10.log        │
│ out log path      │ /var/www/api/logs/out-10.log                       │
│ pid path          │ /home/ubuntu/.pm2/pids/sandbox-10.pid              │
│ interpreter       │ node                                               │
│ interpreter args  │ --trace-warnings                                   │
│ script id         │ 10                                                 │
│ exec cwd          │ /var/www/api                                       │
│ exec mode         │ cluster_mode                                       │
│ node.js version   │ 11.10.0                                            │
│ node env          │ N/A                                                │
│ watch & reload    │ ✔                                                  │
│ unstable restarts │ 0                                                  │
│ created at        │ 2019-11-30T10:45:14.704Z

Reproduce the problem with the shown PM2 configuration and /var/www/api/index.js, then change index.js while watching the PM2 status and logs. Trace the watch and reload behavior from this configuration; done means an index.js change reliably triggers an application restart.