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=184.108.40.206 out-interface=WAN2
/ip firewall nat add action=src-nat chain=srcnat disabled=no src-address=10.5.50.0/24 to-addresses=220.127.116.11
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)