Looks like they'll have to offer you guys an IPv4 routed subnet to work around the issue 😉
There's nothing wrong with the route table entry itself, that is just convention and traffic is pointed to the router itself but there must be some sort of filtering happening after that.
So I wonder what the problem is with the firmware script? Are they trying to do fancy forwarding/filtering like hairpin NAT / NAT loopback (is that a check box feature you can turn off on this router)?