Using additional interface as gateway - weird results

My UT had 2 network interfaces, one for WAN and the other for internal LAN (192.168.1.0/24).

I added another interface to my UT for internal LAN (161.251.62.0/24) to accomodate our customer's requirement for site to site VPN tunnel. This interface has IP address of 161.251.62.8.

I have another router that acts as the VPN router (161.251.62.1).

Both UT and the VPN routers are Gateways.

I changed the IP addresses of workstations of the users servicing the customer to 161.251.62.0/24 network and used 161.251.62.8 (third interface) as the Gateway.

My issue is only 3 simultaneous connections between 161.251.62.0/24 network and the VPN router.

I can see in the logs of the VPN router that 161.251.62.8 is connecting to the VPN. It looks like the IP addresses of the workstations are being NAT'd to 161.251.62.8 even though my NAT policy is 0.0.0.0/0 for interface 161.251.62.8.

I am not sure why UT is acting this way. I thought that since the IP addresses are all private, it will not NAT but in this case, the workstations are like NAT'ing to the IP address of the third interface IP address of 161.251.62.8.

If I make the VPN router my gateway for all those desktops, I don't have any issues with simultaneous connections and I see in the logs that each IP address (161.251.62.0/24) passing thru the tunnel.

Does anyone know what is happening? If it works for 3 workstations, why can't it work for all the workstations in the 161.251.62.0/24 subnet?

Thanks.