Just to follow up on my comments above - having 2 gateways defined in the NICs settings might make them both default?
I am thinking on the DMZ NIC to have a default gateway and then on the LAN NIC have no default gateway but add a static route to the LAN network?
I am thinking on the DMZ NIC to have a default gateway and then on the LAN NIC have no default gateway but add a static route to the LAN network?