Rust assumes stdio handles are opened for synchronous access (as will most other Windows processes). However, when passing ChildStdOut, ChildStdIn or ChildStdErr to a new Command the pipes passed in are asynchronous. For a full test case see src/test/ui-fulldeps/stdio-from.rs but here's a shorter example:

fn try_it(a: &mut Child, b: &mut Child) -> io::Result<()> {
    let mut child = Command::new("application")
        .stdin(a.stdout.take().unwrap())
        .stdout(b.stdin.take().unwrap())
        .spawn()?;

    assert!(child.wait()?.success());
    Ok(())
}

This is related to an older issue: #38811 (comment)

This can cause soundness issues described in #81357.