here is a little code:
<?php
var_dump($this->flash->has('justLogin'));
var_dump($this->flash->getMessages('bbbb'));
var_dump($this->flash->has('justLogin'));
befre running it, I set both justLogin and bbbb. The result on Windows:
true, bbbb, true
while on Linux:
true, bbbb, false
it looks like even if asked for "bbbb" key, getMessages removes everything from the session.