FormData not passing to called XMLHttpRequest as expected

Question

I've given up searching for answers after 4 hours today. I hope someone is able to see something I've done that is just stupid.

I'm using a simple XMLHttpRequest to pass formdata. Appending key and value via for loop.

     let _data = new FormData();
       for (var key in req) {
           var value = req[key];
             _data.append(key, value);
       }

Creating request, open and send

 let xhr = new XMLHttpRequest();
 xhr.open("POST", qb_url, true); 
 xhr.send(_data);

But the formdata is not appearing as valid key values when it hit the PHP page. I appear to get a array with one string encompassing everything, without the parsed key/value when I var_dump $_POST in PHP:

array(1) {
  ["------WebKitFormBoundaryI9nfSkkAwYNXDddt
Content-Disposition:_form-data;_name"]=>
  string(173) ""i"

6
------WebKitFormBoundaryI9nfSkkAwYNXDddt
Content-Disposition: form-data; name="comprobante"

5.060308220031026e 49
------WebKitFormBoundaryI9nfSkkAwYNXDddt--
"
}

Similar/strange output in debugger in Chrome: https://drive.google.com/file/d/12IEDmctiYChcoeGgZjVPluZaGHotOu_t/view?usp=sharing