Module MOX C no dhcp relay possible? Same setup works on MOX E module


i got the MOX with this modules A+B+E+C. Module A is connected to a Fritzbox 7590.
The Fritzbox (FB) is my home-LAN/Wifi device To separate some special devices from my LAN i use the MOX. Special devices are Helium Miner, Homeoffice Laptop from work for example.

Factory setup for br-LAN was (MOX E+MOX C) Port 1-12, one Subnet. I changed this setup:

MOX E - Port 1 - br-PortMiner, dedicated Port, subnet to a Helium Miner, VPN-policy routing to an external VPN server, works fine.
MOX E - Port 2 - dedicated Port,, gw IP is my FB, works fine.
MOX E - Port 3 - 8 br-LAN,, works fine.
MOX C - Port 9 - 12 br-FBLAN, dhcp relay to my FB. I tried to extend the LAN Ports of my FB, doesn’t work.
I setup the br-FBLAN interface with static IP from Fritzbox LAN, gw is FB. Additional i setup the FBRELAY as Relay-Bridge, briding between br-FBLAN and WAN. This setup just don’t work.

If i switch the Ports from MOX C module to MOX E module, the same br-FBLAN/FBRELAY setup works fine. Firewall settings are the same. I just switched the Ports from module C to E. Is this a known bug of the MOX C module? Is dhcp relay not working with the MOX C module?