Can't configure wait_ready

What's going wrong?

Most likely me not understanding how to use wait_ready.
In this example app I've added 15s delay to start to simulate async loading of assets on load.
After starting the app I run pm2 reload pm2test and I would expect the app to be accessible through the whole time.
Instead there is a time window when I can't connect until the app loads fully

How could we reproduce this issue?

this is my config file:

module.exports = {
    apps: [
        {
            name: "pm2test",
            script: "dist/pm2ReloadTest.js",
            instances: "3",
            exec_mode: "cluster",
            autorestart: true,
            time: true,
            wait_ready: true,
           
        },
    ],
};

This is code for app

import express from "express";

const app = express();
const port = 3000;

app.get("/", (req, res) => {
    console.log("GET /");
    res.send("Hello World");
});

let server;
new Promise((resolve) => {
    setTimeout(resolve, 15000);
}).then(() => {
    server = app.listen(port, async () => {
        console.log(`Example app listening at http://localhost:${port}`);

        process.send("ready");
        console.log("ready");
    });
});

process.on("SIGTERM", () => {
    console.log("SIGTERM signal received: closing HTTP server");
    server.close(() => {
        console.log("HTTP server closed");
    });
});

Supporting information

Output from pm2 logs after running pm2 reload pm2test

PM2             | App [pm2test:0] starting in -cluster mode-
PM2             | App [pm2test:1] starting in -cluster mode-
PM2             | App [pm2test:0] online
PM2             | App [pm2test:1] online
PM2             | Stopping app:pm2test id:_old_0
PM2             | Stopping app:pm2test id:_old_1
PM2             | App name:pm2test id:_old_0 disconnected
PM2             | App [pm2test:_old_0] exited with code [0] via signal [SIGINT]
PM2             | App name:pm2test id:_old_1 disconnected
PM2             | App [pm2test:_old_1] exited with code [0] via signal [SIGINT]
PM2             | pid=78393 msg=process killed
PM2             | pid=78394 msg=process killed
PM2             | App [pm2test:2] starting in -cluster mode-
PM2             | App [pm2test:2] online
PM2             | Stopping app:pm2test id:_old_2
PM2             | App name:pm2test id:_old_2 disconnected
PM2             | App [pm2test:_old_2] exited with code [0] via signal [SIGINT]
PM2             | pid=79304 msg=process killed
0|pm2test       | 2023-07-06T10:00:35: Example app listening at http://localhost:3000
1|pm2test       | 2023-07-06T10:00:35: Example app listening at http://localhost:3000
1|pm2test       | 2023-07-06T10:00:35: ready
0|pm2test       | 2023-07-06T10:00:35: ready
2|pm2test       | 2023-07-06T10:00:41: Example app listening at http://localhost:3000
2|pm2test       | 2023-07-06T10:00:41: ready

$ pm2 report

PM2 | 2023-07-06T09:14:42: PM2 log: App [pm2test:_old_2] exited with code [0] via signal [SIGINT]
PM2 | 2023-07-06T09:14:42: PM2 log: pid=74020 msg=process killed
PM2 | 2023-07-06T10:00:20: PM2 log: App [pm2test:0] starting in -cluster mode-
PM2 | 2023-07-06T10:00:20: PM2 log: App [pm2test:1] starting in -cluster mode-
PM2 | 2023-07-06T10:00:23: PM2 log: App [pm2test:0] online
PM2 | 2023-07-06T10:00:23: PM2 log: App [pm2test:1] online
PM2 | 2023-07-06T10:00:26: PM2 log: Stopping app:pm2test id:_old_0
PM2 | 2023-07-06T10:00:26: PM2 log: Stopping app:pm2test id:_old_1
PM2 | 2023-07-06T10:00:26: PM2 log: App name:pm2test id:_old_0 disconnected
PM2 | 2023-07-06T10:00:26: PM2 log: App [pm2test:_old_0] exited with code [0] via signal [SIGINT]
PM2 | 2023-07-06T10:00:26: PM2 log: App name:pm2test id:_old_1 disconnected
PM2 | 2023-07-06T10:00:26: PM2 log: App [pm2test:_old_1] exited with code [0] via signal [SIGINT]
PM2 | 2023-07-06T10:00:26: PM2 log: pid=78393 msg=process killed
PM2 | 2023-07-06T10:00:26: PM2 log: pid=78394 msg=process killed
PM2 | 2023-07-06T10:00:26: PM2 log: App [pm2test:2] starting in -cluster mode-
PM2 | 2023-07-06T10:00:29: PM2 log: App [pm2test:2] online
PM2 | 2023-07-06T10:00:32: PM2 log: Stopping app:pm2test id:_old_2
PM2 | 2023-07-06T10:00:32: PM2 log: App name:pm2test id:_old_2 disconnected
PM2 | 2023-07-06T10:00:32: PM2 log: App [pm2test:_old_2] exited with code [0] via signal [SIGINT]
PM2 | 2023-07-06T10:00:32: PM2 log: pid=79304 msg=process killed

Reproduce the reload with the shown PM2 configuration and delayed Express app, using the PM2 logs and report output as the starting evidence. Trace the reload and wait_ready handling in the PM2 code, then verify that reloading all three instances does not leave a connection gap before the replacement workers are ready.