I’d run IPFire on a vintage 2005 PC with 4 NICs, assigned to Red, Green, Blue and Orange. I had no problem there.
I’ve started running IPFire on a not quite as old PC with 5 NICs. I assigned 4 to Red, Green, Blue and Orange. One NIC was left unassigned. I experienced Red becoming unassigned.
I’ve now assigned Green to 2 NICs in Bridge mode using the Zone Configuration menu, leaving no NICs unassigned. So far Red has remained assigned to the NIC I had selected.
I’ve run it that way, with all NICs assigned, for a very short time, with just a couple of machine reboots as tests. So it’s not been an exhaustive test. But so far, so good.