Here are a few options I can think of. Some of these may not apply to you but I'm saying it anyways for whoever reads this.
1) Disable UPnP (not recommended if you use it for port forwarding on your VPN. You can disable it if your VPN doesn't support UPnP port forwarding.)
2) The way your VPN provider sets up their VPN needs to be adjusted. The reason this is happening is because of how they add the routes. The default gateway is still set as your home router(check with "ipconfig" or "route -4 print") In this case, Tixati is using home router's IP for UPnP and not the VPN network's default gateway.
3) Find a program that let's you specify UPnP settings like the port to forward and the gateway IP. (I haven't tested it so I don't know if this one works: https://sourceforge.net/projects/upnp-portmapper/
4) Run your own wireguard or openvpn VPN server. (Advanced and not recommended but you can setup port forwarding this way.) https://forum.tixati.com/offtopic/337