I got a request for a PtP bridge supporting QinQ with MTU=1518. I was pretty sure I had done this before and thought this was simple, but I might be in over my head.

Thought a simple bridge between two M1515N would be all that is needed, I have sold simple bridges for house-to-house connections and those works out well. Bridge eth1 with wpci1 on each unit. But when I try to change MTU from 1500 to 1518, connection is lost.
I also tried to configure VDS with MTU=1518 and even 1522, without success. This is WAR1b with

I can ping 1518 bytes (and larger) over a standard MTU=1500 bridge, but I guess that is not really sufficient test here? It can't be that easy? How can I get this working, and how do I test (without a vlan and certainly without any QinQ equipment)?