Unable to detect some traffic

TLDR

I set up an iperf3 on a remote machine with iperf3 -s, and connected to it on my local box with iperf3 -c <ADDR>. bandwhich is able to detect the upload traffic as expected.

However, if I connect to it with iperf3 -R -c <ADDR>, so that the server is sending data to the client instead, bandwhich shows very little traffic (which I suppose is the control channel).

Screenshots

Here you can see bandwhich side-by-side with btop, which is able to detect both upload and download traffic correctly. There's obviously something wrong.