You actually have two sessions here. You are sending objects associated with the session in the activateRegistrations method in the context for the trigger. In the BibsAssignerJob you get a new session created specifically for that execution.

A common way to handle this is to first save the objects in the activateRegistrations method and only enclose a list of the objects id's within the context to BibsAssignerJob.triggerNow. In the BibsAssignerJob you'll then have to load those objects again within that session - Registration.get() - and you will be able to continue work with them as expected.