It looks clear to me that the icmp responses are being send back to A.
11:46:49.528951 IP MachineC > 10.0.73.11: ICMP echo reply, id 36931, seq 1, length 64
Do the test again, but this time, open up another connection to machine A and start a tcpdump on it and filter icmp. Do you see icmp responses on machine A when you have tcpdump running on it?
to be sure, run this on machine A and B and show us the results: