OSX Forticlient doesn't add a new route for sslvpn server?

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Hi there, 

We are using a 100D as VPN box, fortiOS 5.2.0. With multiWAN (ISP1 and ISP2) 

For some reason , we used split tunnel, and we also push 0.0.0.0/1  and 128.0.0.0/1  to some specific users/groups. 

And when it comes to Max OSX forticlient. It acts super weird, when it connects to VPN via WAN1's IP, everything works fine. 

And we can find a new route added by forticlient with command "netstat -nr | grep WAN1's_IP "

Somehow, if we connect the VPN via WAN2. It doesn't add the route,and the VPN tunnel alive for only 1 sec, then lost connection to the remote gateway.  I check the forticlient log, and there is no "Add a new route for sslvpn server", which is expected. 

May I know why it has different behaviour with 2 WAN interface. Bug or misconfig?