native vlan (untagged) not forwarded from bridge

[void]

Hi,

I am running Debian Bullseye with proxmox 7.
When creating a VM with a tagged interface everything works fine.
Untagged traffic gets lost on the PVE Machine, even though PVE-FORWARD all any any is hit.
FW Disabled on Host and VM Level.
iptables FORWARD chain was set to policy DROP, setting it to ACCEPT resolved it temporarily..
Where to tune?

BR
Matthias