CAN forwarding of custom device


I want to know how it is possible to let the autopilot forward every CAN message of a device connected to it via MAVLink to my ground station.

I have a custom device that sends messages that are conform to the CAN protocol. It is connected to the CAN_2 interface of my autopilot. CAN_P2 is set to use CAN_D1, and has the same Bitrate as my device. CAN_D1 is enabled and uses DroneCAN protocol.
However using the command MAV_CMD_CAN_FORWARD (32000), it appears, that the autopilot either can’t see the messages, or it just doesn’t forward them, because the messages that my custom device sends do not appear, only those of built in components(like GPS). Is there any way to verify if ardupilot even receives those messages?

Apart from that, the autopilot stops forwarding the messages after about 10seconds and I have to continously send the command in this interval in order for the autopilot to continue forwarding messages. Is this intended behavior or is this a bug?