I also had this problem.

I am using Debian 9.1 and PHP 7.1.8.

Here is some sample code to reproduce the problem:
----------

class topClass
{
	public $status;
	public $baseRef;
}
class nestedClass1
{
	public $statusDetail;
	public $isSuccess = 1;
}
class nestedClass2
{
	public $internalId = 1;
	public $externalId = 3;
	public $typeId = 10;
	public $name;
}

$testObject = new topClass();
$testObject->status = new nestedClass1();
$testObject->baseRef = new nestedClass2();

$flatObject = new stdClass();

$iterator = new RecursiveIteratorIterator(
	new RecursiveArrayIterator($testObject)
);

foreach ($iterator as $key => $value) {
	$flatObject->$key = $value;
}

var_dump($flatObject);
----------

This is the result with the versions I tested.
PHP 5.2.17 and also 5.4.45:
object(stdClass)#99 (6) {
  ["statusDetail"]=>
  NULL
  ["isSuccess"]=>
  int(1)
  ["internalId"]=>
  int(1)
  ["externalId"]=>
  int(3)
  ["typeId"]=>
  int(10)
  ["name"]=>
  NULL
}


PHP 7.1.8:
object(stdClass)#99 (2) {
  ["status"]=>
  object(nestedClass1)#97 (2) {
    ["statusDetail"]=>
    NULL
    ["isSuccess"]=>
    int(1)
  }
  ["baseRef"]=>
  object(nestedClass2)#98 (4) {
    ["internalId"]=>
    int(1)
    ["externalId"]=>
    int(3)
    ["typeId"]=>
    int(10)
    ["name"]=>
    NULL
  }
}

It seems that this fix caused a BC break from php 7.1.13. See the following gist on how to reproduce: https://gist.github.com/andriesss/b22b4057bd7e0563f4d21b7b2cfd6a09