Virtual wire pair before wan interface blocks firewall traffic, proxy traffic works fine

I have a working fortigate v7.2.2 with:
wan1 = Internet

interface 5 = proxy clients

interface 6 = connection from lan on port 6 to internet wan1  through an allow all policy

Everything works great.

Now I created a virtual wire pair (port 3&4) and I put an "allow all policy" on it and enabled IPS with a default_pass_all policy.
I use this wire pair as a connection between the isp modem and the wan 1 interface.
so the flow is now isp -> lan3 ...... lan4>wan1
with this wire pair inserted before the wan1 connection, the web proxy still works, but there is no traffic from the lan connected to interface 6 to the internet.

So it looks like the virtual wire pair is blocking something, I tried both the VLAN Wildcard option on and off, but both gave the same result. I even removed the IPS inspection on the wire pair, but no luck.

What am I missing? Why does the insertion of the virtual wire pair before wan1 inhibit traffic from the lan connected to interface 6. I had expected the virtual wire pair to be fully transparent, but apparently it isn't.