How to flush an expressjs response partially piped stream?

Question

When I run this sample on express v4.18.2, I don't see any of the response data in the Chrome Debugger Network Tab nor in Fiddler until after the 15 second mark. This is despite that "First part was piped" / "First part was closed" shows up immediately in the server side console. How can I force the response stream to be flushed earlier without ending the response, please?

import { Readable } from "stream";

app.get("/api/data/multipart", (request, response, next) => {
    response.useChunkedEncodingByDefault = true;
    response.status(200);

    const p1 = Readable.from("Foo".repeat(1000))
    p1.on('close', () => {
        console.log(`First part was closed`);
    });
    p1.pipe(response, { end: false });

    console.log(`First part was piped`);

    global.setTimeout(() => {
        Readable.from("Bar".repeat(1000)).pipe(response, { end: true });

        console.log(`Second part was piped`);
        next();
    }, 15 * 1000);
});

Output:

First part was piped << Immediate
First part was closed << Immediate
Second part was piped << ~15s