Vercel "/" route returns server source code

I am hosting a REST API in Vercel (serverless). The root directry looks like this:

+ myapp_root_dir
|-+ api
| |-+ public
| | |-- index.html
| | |-- layout.css
| | |-- main.js
| |-- index.js
|-- vercel.json

This is the server code (index.js):

import express form 'express'
import session from 'express-session';
import cors from 'cors'
import path from 'path';
import { fileURLToPath } from 'url';

const __filename = fileURLToPath(import.meta.url);
const __dirname = path.dirname(__filename);

const app = express()
app.use(session({
    secret: 'somesecrethere',
    resave: true,
    saveUninitialized: true,
    cookie: {
        secure: false,
    }
}))
app.use(cors())
app.use(express.json())
app.use(express.urlencoded())
app.use(express.static("public", {
    index: false
}))

app.get("/", (req, res) => {
    res.sendFile(__dirname + "/public/index.html")
})

app.get("/time", (req, res) => {
    res.send(new Date().getTime())
})

export default app

And this is the vercel.json config file

{
    "rewrites": [
        { "source": "/time", "destination": "/api" },
        { "source": "/", "destination": "/api" }
    ]
}

The /time endpoint is working correctly and returns the current time. However, the / endpoint returns the server code (the contents of index.js).

Edit: I was able to get around this issue by moving the / handler to a new route (home) and placing a redirect in vercel.json:

{
    "rewrites": [...],
    "redirects": [
        { "source": "/", "destination": "/home" }
    ]
}

What is so weird is that I have another app with the same vercel.json config that is working perfectly well. So what is going wrong here?