I hit this too, as have many others. The consensus has been that the DHCP relay needs to listen on all interfaces where DHCP is to be forwarded, in both directions. This means the interface through which the DHCP server resides does need to be included.
It is not intuitive but it has been that way for quite some time. The "bug" actually resides in the upstream dhcrelay, which is used by many distributions. Reportedly this was fixed about a year ago in an at-thetime "unstable" (development) release of isc-dhcp. I don't know if that fix ever made it to EdgeOS.
