It also seems to be related to Billybob's one here: https://community.sophos.com/products/unified-threat-management/astaroorg/f/98/t/68433
I've made another screenshot:

I only have two interfaces (ETH0 internal, ETH1 external). External connections from inside LAN to outside WAN should therefore be displayed in both monitors, right?
As you can see, external connections are shown in the overall monitor, but not on the single interface ETH1 monitor at the same time.
Regards,
Bastian