On the MX when doing dhcp relay via a bridge domain and irb interface you need to have broadcast flood enabled so that all hte bridged interfaces can communicate for the relay sessions.
Enable broadcast on the IRB interface to flood discovery frames from all physical interfaces in the bridge domain. For example,
user@host# edit forwarding-options helpers bootp interface irb.o
broadcast;
server 202.67.4.1;
}
Details are here: