1. PersistentCollection in Doctrine 2.5 makes uses of the $initialized property defined in the AbstractLazyCollection of doctrine\collection (https://github.com/doctrine/doctrine2/blob/master/lib/Doctrine/ORM/PersistentCollection.php#L113)
2. Doctrine 2.5 depends on version ~1.2 of doctrine\collection that marks the $initialized property as private https://github.com/doctrine/collections/blob/v1.2/lib/Doctrine/Common/Collections/AbstractLazyCollection.php
3. Hence writing to the initialized variable fails. And thus code related to initialization fails.
A fix is already applied to doctrine\orm https://github.com/doctrine/collections/commit/e2eef4629349fd847c8e080b8f729ab33e81e10f but is not tagged. Possible fix would be to release a new version of doctrine\collection as 1.2.1.