org.hibernate.jpa.spi.AbstractEntityManagerImpl.joinTransaction(boolean explicitRequest) used to check if a transaction is active and throw a TransactionRequiredException if not. We seemed to lose that with the

			if ( transaction.getJoinStatus() == JoinStatus.NOT_JOINED ) {
				if ( explicitRequest ) {
					throw new TransactionRequiredException( "No active JTA transaction on joinTransaction call" );
				}
				else {
					LOG.debug( "Unable to join JTA transaction" );
					return;
				}

Javadoc for EntityManager.joinTransaction():

void joinTransaction()

Indicate to the entity manager that a JTA transaction is active. This method should be called on a JTA application managed entity manager that was created outside the scope of the active transaction to associate it with the current JTA transaction.

Throws:
TransactionRequiredException - if there is no transaction