Are you using the seam managed persistence context? If so did you start the conversation with FlushMode.MANUAL?
Why do the entities need to be merged? If you are using a conversation scoped SMPC then the entities should already be contained in the entityManager and will be automatically flushed after every request (and most entityManager operations), which is probably causing the behaviour you are seeing.
You spotted my mistake. I accidentally did not use an SMPC. After changing the application, I see now the behaviour I am expecting.
Thank you very much!