XMLHttpRequest.status always 0 in wasm

My JS/QML code makes a simple XMLHttpRequest like this:

var xhr = new XMLHttpRequest();
xhr.onreadystatechange = () => {
    if (xhr.readyState === 4) {
        // here I examine xhr.status
    }
};
xhr.open("GET", "https://somewhere", true);
xhr.responseType = "json";
xhr.send(null);

Which works great on desktop Linux, but when compiled for WebAssembly, xhr.status is always 0 inside the if condition in the code.

Other values (such as xhr.responseText) are set correctly.