Unexpected resource exceeds maximum size error with Request

I am doing a standard Request with the DeepL API, but the request unexpectedly throws a “resource exceeds maximum size” error:

let items = await loadItems();

async function loadItems() {
  let url = "https://api-free.deepl.com/v2/translate";
  let req = new Request(url);
  let body = {"text": "Hello!", "auth_key": "[xxx]", "target_lang": "ES"}
  req.body = JSON.stringify(body);
  let json = await req.loadJSON();
  return json

console.log("this is " + items)

Have played with the DeepL API in Postman with no problems, so I don’t think it’s a problem with the API.

Am I doing something stupid?

Sounds like it might be this iOS constraint.