Why is WordPress serving the 1024px version of my image and not the original on a 1920×1080 screen?

Your image’s src is pointing to the full size image, so it’s not an issue of permalink. The issue happens because of the sizes values. There is a variable that determines the width of your content, which then later WordPress uses to generates the responsive sizes for your images.

This global variable can be set manually in your theme’s functions.php file:

if( !isset($content_width) ) {
    $content_width = 1920;
}

You can tweak with this variable to achieve your desired result.