CDI spec tries to dispose a container-managed entity manager.

Section "3.5.2. Declaring a disposer method" has this example:

public class Resources {

    @PersistenceContext
    @Produces @UserDatabase
    private EntityManager em;
    
    public void close(@Disposes @UserDatabase EntityManager em) {
        em.close();
    }
}

The disposer method above call EntityManager.close() on a container-managed entity manager which result in an IllegalStateException being thrown and the entity manager remains open. This behavior is expected as the JPA specification forbids application code to close a container-managed entity manager. JPA 2.1, section "7.9.1 Container Responsibilities" says:

The container must throw the IllegalStateException if the application calls EntityManager.close on a container-managed entity manager.

This has been proved here and a theoretical walkthough surrounding entity managers combined with CDI scopes is available here.