Hmm…
One quick thought that I have (I may be wrong) is that tcpdump is putting your ethernet interface in promiscuous mode and as a result, your raspy / OpenHab can read the packets originating from the KNXnet/IP Router (multicast)
Without tcpdump running, check the output of netstat -i
see if the flags include “M”
if not, enable multicast: ip link set eth0 multicast on
or set the ethernet interface to always on promiscuous mode: ip link set eth0 promisc on (check for the “P” flag in netstat)
Not sure if this is related to your networking issue…