Why glium `Headless` can not render an image like normal window context?

Question

I am working on an off-screen render program and I use crate glium to do this. I have followed the example of screenshot.rs and this example worked well.

Then I made some change:

The orignal code was

fn main() {
    // building the display, ie. the main object
    let event_loop = glutin::EventsLoop::new();
    let wb = glutin::WindowBuilder::new().with_visible(true);
    let cb = glutin::ContextBuilder::new();
    let display = glium::Display::new(wb, cb, &event_loop).unwrap();

    // building the vertex buffer, which contains all the vertices that we will draw

I grouped these codes into a function:

fn main() {
    // building the display, ie. the main object
    let event_loop = glutin::EventsLoop::new();
    let display = build_display((128,128), &event_loop);

    // building the vertex buffer, which contains all the vertices that we will draw

pub fn build_display(size: (u32, u32), event_loop: &glutin::EventsLoop) -> glium::Display {
    let version = parse_version(); //this will return `OpenGL 3.3`
    let wb = glutin::WindowBuilder::new()
        .with_visibility(false)
        .with_dimensions(glutin::dpi::LogicalSize::from(size));
    let cb = glutin::ContextBuilder::new()
        .with_gl(version);
    glium::Display::new(wb, cb, &event_loop).unwrap()
}

After this modification, the program still worked well. So I continued to add the headless-context:

fn main() {
    // building the display, ie. the main object
    let event_loop = glutin::EventsLoop::new();
    let display = build_display_headless((128,128), &event_loop);

    // building the vertex buffer, which contains all the vertices that we will draw

pub fn build_display_headless(size: (u32, u32), event_loop: &glutin::EventsLoop) -> glium::HeadlessRenderer {

    let version = parse_version();  // this will return `OpenGL 3.3`
    let ctx = glutin::ContextBuilder::new()
        .with_gl(version)
        .build_headless(&event_loop, glutin::dpi::PhysicalSize::from(size))
        .expect("1");
    //let ctx = unsafe { ctx.make_current().expect("3") };
    glium::HeadlessRenderer::new(ctx).expect("4")
}

But this time, the program did not work. There was no panic during the running, but the output image was empty with black, and its size was not 128x128 but 800x600.

I have tried to remove the libEGL.dll so that, due to the doc of crate glutin, the function .build_headless will build a window and hide it, just as my function build_display does. However, this failed too. So what can cause this?