One possibility is you've firewall enabled on the sever.
You can try to run this command to verify.
iptables -t nat -L -v
iptables -t mangle -L -v
iptables -t raw -L -v
iptables -t security -L -v
From the output, if you saw something like this
"MASQUERADE all – any any anywhere anywhere"
This means you enabled masquerading for all traffic. This causes local to local traffic also to be source NATed.
