VIP source address question

Hi,

I have configured VIP (1:1 NAT) to allow external SFTP access from the internet into our server. The connection works but why the source address logged on the FTP server is the internal LAN IP address of the Fortigate instead of the actual src add of the SFTP client? Is this a normal behaviour of Fortigate? Is there a way to force the FortiGate not to replace the original src add?   

Thanks in advance.