JTA spec states in 3.3.2 Transaction Synchronization:

The Synchronization.beforeCompletion method is called prior to the
start of the two-phase transaction commit process. This call is executed with
the transaction context of the transaction that is being committed.

I guess the event is fired too eagerly in the test because I think the synchronization callback is called once the method (EjbTestBean#initTransaction in this case) was executed.