The problem is that Styles like the SymfonyStyle do not use stderr when appropriate.
Errors and diagnostic and interactive messages that are not meant as actual ouput of the command should go to stderr. For background info see http://www.jstorimer.com/blogs/workingwithcode/7766119-when-to-use-stderr-instead-of-stdout

For this we merged #15795 to help people use the right one by default. But this will not take effect when using the styles currently, e.g. when using askQuestion in https://github.com/symfony/symfony/blob/2.8/src/Symfony/Component/Console/Style/SymfonyStyle.php#L330