DNS Reverse Lookup Broken?

Hello,

On my RAX120 router, I've configured two internal DNS IPv4 servers on my local network plus a 3rd external DNS server which is a primary for my ISP.  Forward DNS lookups work fine for any host which is configured by DHCP: the router seems to properly forward to one or both internal DNS servers but reverse lookups always return NXDOMAIN errors.  I know reverse lookups work fine when I set the DNS resolver to reverse lookup directly to each DNS server.  It must be that reverse lookups when forwarded from the router are broken.  Anyone else experienced this?