pm2 watch argument doesn't watch the files - node.js

Question

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 doesn'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