RestTemplate POST throws 400: Bad Request while calling GraphQL Endpoint

I'm trying to call the Shopify Partner API to validate the authentication. I have tried multiple ways that were successful, for instance, I received a successful response with Postman:

curl --location --request POST 'https://partners.shopify.com/2XXXXX8/api/2021-10/graphql.json' \
--header 'X-Shopify-Access-Token: prtapi_...' \
--header 'Cookie: _partners_session=a4a...; monorail_user_token=3648077d-...'

I received a successful response when I used org.apache.http.impl.client.CloseableHttpClient;:

    try (CloseableHttpClient httpClient = HttpClients.createDefault()) {
      HttpPost httpPost = new HttpPost(restUrl);
      httpPost.addHeader("X-Shopify-Access-Token", accessToken);

      try (CloseableHttpResponse httpResponse = httpClient.execute(httpPost)) {
        int statusCode = httpResponse.getStatusLine().getStatusCode();
        if (statusCode != 200) {
          log.warn("Auth Failed, Response Status:: " + statusCode);
        }
      }
    }

and I also received a successful response when I used okhttp3.OkHttpClient;

    RequestBody reqbody = RequestBody.create(null, new byte[0]);
    OkHttpClient client = new OkHttpClient();
    Request request =
        new Request.Builder()
            .url(restUrl)
            .addHeader("X-Shopify-Access-Token", accessToken)
            .post(reqbody)
            .build();

    try {
      Response response = client.newCall(request).execute();
    } catch (IOException e) {
      e.printStackTrace();
    }

But the problem is that I am bound to use only org.springframework.web.client.RestTemplate; I have tried multiple ways to achieve that but for some reason, it's not working:

    HttpHeaders headers = new HttpHeaders();
    headers.set("X-Shopify-Access-Token", accessToken);
    headers.add("X-Shopify-Access-Token", accessToken); // Just in case

    RequestEntity requestEntity = new RequestEntity(headers, HttpMethod.POST, URI);
    ResponseEntity<String> res = restTemplate.exchange(requestEntity, String.class); // Fails

    MediaType mediaType = MediaType.TEXT_PLAIN;
    RequestEntity requestEntity = RequestEntity.post(uri).headers(headers).contentType(mediaType).build();
    ResponseEntity<String> res = restTemplate.exchange(requestEntity, String.class); // Fails

    headers.setContentType(MediaType.TEXT_PLAIN);
    HttpEntity<String> entity = new HttpEntity<>("", headers);
    String res = restTemplate.postForObject(uri, entity, String.class); // Fails

For all the requests above, I'm getting 400 : [Bad Request]. What am I missing? I am also adding Postman's OKHttp version for reference:

OkHttpClient client = new OkHttpClient().newBuilder()
  .build();
MediaType mediaType = MediaType.parse("text/plain");
RequestBody body = RequestBody.create(mediaType, "");
Request request = new Request.Builder()
  .url("https://partners.shopify.com/2XXXXX8/api/2021-10/graphql.json")
  .method("POST", body)
  .addHeader("X-Shopify-Access-Token", "prtapi_...")
  .addHeader("Cookie", "_partners_session=a4a...; monorail_user_token=3648077d-...")
  .build();
Response response = client.newCall(request).execute();