On the WAN, DHCP only works once I run tcpdump on the vlan

[ErnestBunbury]

Hello
OPNSense V22.1.6 no plugin, running on Protectli VP2410 Coreboot, 4GB RAM 8GB eMMC
On the WAN, to receive an IP, I need to use a VLAN tagged 100.
If I just do that, it wont work, without giving an error message.
But if I run tcpdump on the vlan from ssh, then suddenly everything flow perfectly.
The second I stop tcpdump, nothing.
I tried reseting to factory default, I tried other interfaces, I tried reinstalling, no change.
What am I missing ?
Thanks for your help.