I have the same issue as well. I was able to find a work around.
Once you connect to your VPN via Forticlient, on the main window it will tell you your assigned IP. Take note of that.
Go into your network adapters and find the Fortinet SSL Virtual Ethernet Adapter:
Highlight IPv4 and open properties.
Update the static IP with the one given in the Forticlient window.
It seems like the Forticlient software is not passing the IP over to the virtual adapter when connecting. Doing this manually will fix it. It's likely you'll have to do this each time you connect until there's a permanent fix.