In that case the conclusion is that PPP tunnel is the problem, not NAT.In my own experience, the issue persists while NAT is not used.
/ip firewall connection tracking set enabled=no /ip firewall connection tracking set enabled=yes
/ip firewall filter add action=drop chain=output comment="gateway control" disabled=yes dst-address=220.127.116.11 out-interface=WAN2
/ip firewall nat add action=src-nat chain=srcnat disabled=no src-address=10.5.50.0/24 to-addresses=18.104.22.168
tunnel mode will be fully disabled in next version of RouterOS as it should have been. Try to use other tunnel type is you need secure connection (PPTP for example)