Possible bug with Static routing IP range - RT-AC68U

[Ugh]

I'm having trouble setting up static routes for an address range for an RT-AC68U, and wonder if this is a bug or not. This is with firmware 3.0.0.4.385_20633

My IP addresses below have been changed to protect my network, but this is the gist of it.

I have a double-NAT situation, so both LAN and WAN are on non-routable addresses.

WAN subnet is 10.10.10.x, 255.255.255.0
LAN subnet is 192.168.1.x, 255.255.255.0

If I set up a static route as follows, it works OK:
Host IP:10.10.10.50 Netmask: 255.255.255.255 Gateway:192.168.1.111 Metric=2 Interface=LAN

However, if I select an address range by loosening up the Netmask, like this, it doesn't work:
10.10.10.50 255.255.255.248 192.168.1.111 Metric=2 Interface=LAN

Playing around with it, the static route doesn't seem to work at all unless I select just a single client (i.e. 255.255.255.255).

Is this a bug? Can anyone replicate it? Can you please explain why this doesn't work as I expect, and if it's not a bug, why is the Netmask field in there at all?

Thanks,
Alan