Doctrine - Managed Entity - Fetched Entites are not managed by default
Ok. I found the solution. I used a special dependency injection "technique" in typo3 extbase.If you specify your dependencies in as arguments of __construct() it will inject the service without any further configuration. So i just assumed, that it uses an internal 'service bus' to serve all dependencies with the same object instances, but instead it created one for each dependency request.
So i got three diffrent entity managers and obviously somehow not managed entities.. sorry to bothered you folks.