[FS#151] RST packet as response to SYN is not forwarded to sender?

[LEDE Bugs]

A new Flyspray task has been opened.  Details are below. 

User who did this - Github Issues (github-import) 

Attached to Project - LEDE Project
Summary - RST packet as response to SYN is not forwarded to sender?
Task Type - Bug Report
Category - Base system
Status - Unconfirmed
Assigned To - 
Operating System - All
Severity - Low
Priority - Very Low
Reported Version - Trunk
Due in Version - Undecided
Due Date - Undecided
Details - Github issue # reported by: https://github.com/xnoreq

Following setup:
A host on the LAN sends a SYN packet through the router to a remote server. The server rejects it with tcp-reset.
tcpdump on the router shows the SYN packet being sent and the RST,ACK packet being received over eth0.2 (WAN interface), but the packet never makes it to the host on the LAN.

In iptables the reset packet shows up in -t mangle PREROUTING and then INPUT instead of FORWARD. Why?

I've tried disabling the firewall (all iptables flushed) and just added a single MASQUERADE rule ... same result.
This is on 15.05.1 with Linux 3.18.23.

More information can be found at the following URL:
https://bugs.lede-project.org/index.php?do=details&task_id=151