Fortigate sends Reset to Server after receiving Fin from Client

I'm troubleshooting an issue and I'm not sure if I'm experiencing normal FortiGate behavior.

I have clients who are accessing a server on the internet. At any given point, if the client's send a FIN,ACK to the Server (ie: The fortigate's LAN interface receives a FIN,ACK), the fortigate sends a RST,ACK to the server instead of the original FIN,ACK. 

Does anyone know if this behavior is normal? The session is not timing out, it's just a normal close.