Replying to both of your comments:
These are captured on both my gateway and the Apache LXC container. The captured packets are identical as far as tcpdump is aware on both of these systems. As far as I can tell, unless there are shenanigans at the firewall WAN NIC, this is how the packets arrive to my firewall.
And I don't think this is asymmetrical routing if I understand it correctly, as I only have one firewall. My interfaces are configured correctly according to that netgate article.