stdio[3 + n] not working with child_process

[Clemens-E]

What version of Bun is running?

1.1.8-canary.1+88468a2c2

What platform is your computer?

Microsoft Windows NT 10.0.19045.0 x64

What steps can reproduce the bug?

It's possible to add extra stdio streams when spawning a child, bun provides those streams, but no message is received.
Nodejs receives them without issues

To reproduce:
child.js

import { createWriteStream } from 'fs';

const stdio3 = createWriteStream(null, { fd: 3 });
console.log('stdout: createWriteStream fd: 3')
stdio3.write('hi from bun! using fd 3')

index.js

import { spawn } from 'child_process';

const child = spawn('bun', ['src/child.js'], {
    stdio: ['inherit', 'inherit', 'inherit', 'pipe']
});

console.log(`${child.stdio.length} available file descriptors`)

child.stdio[3].on('data', d => console.log(`fd 3: ${d.toString()}`))

What is the expected behavior?

expected nodejs behavior:

$ node src/index.js 
4 available file descriptors
stdout: createWriteStream fd: 3
fd 3: hi from bun! using fd 3

What do you see instead?

$ bun src/index.js
4 available file descriptors
stdout: createWriteStream fd: 3

Additional information

Using Bun.spawn, we see that
bunSpawn.stdio (which is not documented, at least not in the types)
provides an integer at the position 4: [ null, null, null, 172 ]
I don't know what to do with this value, as passing it to createReadStream(null, {fd:bunSpawn.stdio[3]}) yields no data and instantly closes

This seems related to #6240