IPV6 Neighbour table with IF forwarding?

[pbtgit]

I have a setup on NRF5340 based on Zephyr 3.2.
One device acts like a "border router" - 2 network IF's with 2 local IPV6 address spaces.
(One address space for my sensor network and one address space for a host computer)

The external network IF is based upon USB-ETH with IPV6 ND activated.
(This is more or less required unless I disable ND in the host computer but I'm not sure that is a good idea)

Now the observation is the Neighbour table in the "BR" does not get populated when another node tries to send through the BR to host computer - it works only when there has been traffic between BR and the host initiated from the BR or host:
Then neighbour table is updated and traffic from other nodes works.

To me it seems like the Neighbour discovery does not work properly?

Best
Peter