I've bumped into the same issue on PHP 7.0.1 (cli).
On top of this I noticed while using the FileGenerator::fromReflectedFileName() method, that it also changes Interface names that are included with a use statement into the FQN.

For example:

Source

<?php

namespace Testing;

use Phpro\SoapClient\Type\ResultInterface;

class ArrayOfForecast implements ResultInterface
{
// OLD BODY
}

Destination:

<?php

namespace Testing;

use Phpro\SoapClient\Type\ResultInterface;

class ArrayOfForecast implements Phpro\SoapClient\Type\ResultInterface
{
// NEW BODY
}

As you can see the trailing slash is missing and it should rather use the short name of the interface like in the source file.

In the ClassGenerator, you do have access to the use statements as far as I can tell from the code. Is this something that is possible?