Hi Roger, 

Since last 1 week I have been watching this thread and it just keeps getting interesting. As James and Dimitry mentioned earlier that wherever "ovn-cms-options=enable-chassis-as-gw" is set that node will act like gateway of all VM and It will handle all external in/out traffic. 

Can you post output of ovs-vsctl show and ovs-vsctl list Open_vSwitch of your gateway node? 

I will see if I can set up my lab to create a scenario similar to you to untangle this mistry.