The reason is guest network should be isolated. You can choose your own ip range in Foris, but different from your lan, wan, vpn, like 192.168.y.z: http://192.168.x.y/foris/config/main/lan/ default is 10.a.b.c.
Sorry I can’t help you. I cannot reproduce it so maybe you’ve made some fault in setup. My setup is made in Foris with something like 10.XXX.XXX.X IP address. And I can see guests with right IP under LuCI under Status > Overview in DHCP Leases. Also arp in ssh is right something like br-guest_xxxx.