Multicast over localhost and not ETH0

[deepsurvey]

Hi,
when we run adsbcot set to multicast udp+wo://239.2.3.1:6969 we see the multicast data over localhost and not over our LAN connected NIC.
Is there a (pytak) variable to set the destination network interface?
Funny enough we tried 2 different Raspberry Pi installs and get the same result on both. Wireshark shows all multicast data on 127.0.0.1 and not on eth0.

We must be missing something trivial.