They are probably using proxy arp
That was discouraged a long time ago, for security reasons.
Why would you want that feature ?
If using DHCP the PLC should also accept the def-gw info handed out.
If using Static IP, it's just one more entry to key in.
The whole point of using a firewall is to be "In Control", and not rely on some (could even be a hostile) device, forwarding your packets based on unanswered arp requests.