I have the following routes:

default via 192.168.1.1 dev wlan0 proto dhcp src 192.168.1.147 metric 3003
192.168.1.0/24 dev eth0 proto kernel scope link src 192.168.1.100 dead linkdown
192.168.1.0/24 dev wlan0 proto dhcp scope link src 192.168.1.147 metric 3003

I have the following sysctl settings:

"net.ipv4.conf.all.ignore_routes_with_linkdown" = 1;
"net.ipv4.conf.default.ignore_routes_with_linkdown" = 1;

From the kernel docs: Ignore routes whose link is down when performing a FIB lookup.

% ip r g 192.168.1.1
192.168.1.1 dev wlan0 src 192.168.1.147 uid 1000
    cache
%

eth0 is unplugged. Ping packets to 192.168.1.1 leave and arrive on wlan0 correctly. Despite that, the nixos-fw rpfilter chain drops those packets (checkReversePath = "strict", logReversePathDrops = true).
The relevant reverse path filter rule is fib saddr . mark . iif oif exists accept.

If I remove the routes on eth0, the pings are no longer dropped.

Why does routing work correctly, but the reverse path lookup fails?

      "30-enp1s0" = {
        matchConfig.Name = "enp1s0";
        # Allow baby jumbo frames to theoretically allow
        # PPP-encapsulated packets with 1500 Bytes payload
        # to pass through here.
        linkConfig.MTUBytes = "1512";
        address = [
          "192.168.1.2/24"
        ];
      };

      "30-enp1s0" = {
        matchConfig.Name = "enp1s0";
        # Allow baby jumbo frames to theoretically allow
        # PPP-encapsulated packets with 1500 Bytes payload
        # to pass through here.
        linkConfig.MTUBytes = "1512";
        address = [
          "192.168.1.2/24"
        ];
      };