OPNsense ignoreing ARP table for SOME trafic

[GaardenZwerch]

Hi,
I am pulling my hair here:

• my opnsense box ignores what's in its ARP table for traffic from the box itself. It send the the layer 2 address of another host on the same network.
• for traffic coming in through my opnsense box, for the same destination, it uses the correct layer 2 address.

What can cause this? What can I look for?
Here are some exhibits:
the ARP table
the routing showing the gateway
tcpdump for a ping origination on the opnsense box and for a ping coming through the opnsense box

hardwiring the gates MAC into the arp table doesn't change the behaviour.

Many many thanks for any help
Frank