Go to Policy&Objects > IPv4 > Policies and look for your wan to VLAN/internal policy
Double click it
Set NAT to OFF
NAT or Network Address Translation literally takes the TCP/UDP package, changes the Source Address with a set (or the interfaces) IP address.
This is useful when going into external networks, i.e. when I access a website I want my NAT device to exchange my 192.168.1.1 IP with my external 126.96.36.199 IP or the webserver will ignore my request since 192.168.1.1 is a private IP and the package (martian package) literally can't find it's way back to me and will be discarded.
But on external > Internal policies it's best to leave it off, so the original IP will be transmitted.