Hi there,
here is the patch proposed:

- json_encode($filesInfo, defined('JSON_PRETTY_PRINT') ? JSON_PRETTY_PRINT | JSON_UNESCAPED_SLASHES : 0);
+ json_encode($filesInfo, defined('JSON_PRETTY_PRINT') ? JSON_PRETTY_PRINT |  JSON_UNESCAPED_SLASHES : 0);

php-cs-fixer tries to add an additional space before JSON constants separated by a pipe, giving to spaces rather than one.

Here is the analysis ran by fabbot http://fabbot.io/report/symfony/symfony/19922/04275945efe1f0d035f1744543138804e3826c3e in symfony/symfony#19922.

Not sure about which fixer causes this as I don't know which are used or not.