Description:
------------
Tested with: PHP 7.1.10, PHP 5.6.31 and curl 7.56.0
The script works with curl 7.55.0 correctly.

Test script:
---------------
curl.php
--------------------------------------------------------------------------------------
<?php

//generate string data
function getStr($num) {

  $bstring = 'abcdefghijklmnopqrstuvxyz0123456789';
  $str = '';

  for($i = 0; $i < $num; $i++) {

    $str .= $bstring;

  }

  return $str;

}

$json = [
  'json1' => getStr(200),
  'json2' => getStr(200),
  'json3' => getStr(200),
  'json4' => getStr(200),
];

$post = [
  'test1' => 'a',
  'test2' => 'b',
  'test3' => 'c',
  'test4' => json_encode($json) //or use print_r or var_dump with return parameter
];

$ch = curl_init();

//send encoded json-data to service

curl_setopt($ch, CURLOPT_URL, 'https://localhost/service.php');
curl_setopt($ch, CURLOPT_POSTFIELDS, $post);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);

$result = curl_exec ($ch);

curl_close($ch);

var_dump($result);

?>

service.php
--------------------------------------------------------------------------------------
<?php

//get the post data and write into request.log

$result = file_put_contents('request.log', print_r($_REQUEST, true));
var_dump($result);

//the content of $_REQUEST is wrong, json1 is repeated, content is cutted off
//expected result: correct content in $_REQUEST

?>

Expected result:
----------------
Correct post data in $_REQUEST.

Actual result:
--------------
the content of $_REQUEST is wrong, json1 is repeated, content is cutted off, the post data is corrupt