NAT loopback debugging help wanted

So I found out the NAT loopback works if the connection from the router to the server being accessed is via one of the ethernet ports on the device (rather than through an intermediate switch).  I also found out that this is likely due to the

bridge-nf-call-iptables

setting, based on information in a forum of a different consumer device.

That is, if you issue:

echo 0 > /proc/sys/net/bridge/bridge-nf-call-iptables

it will work (just as if you turn on promisc mode on br-lan).

So this is interesting.  The RBR 750 uses Linux's virtual bridging (a software implementation of Layer 2 switching) to connect the 3 physical ethernet ports (eth1, eth2, eth3) and the wireless interfaces into a single bridge interface. The problem is not the type or kind of switch, it's the fact that those packets cannot be sent directly to their layer-2 receiver.  I don't fully understand it yet.