Quick update, the latest firmware TL-MR6400(EU)_V2_170912 resolved this issue for me https://www.tp-link.com/au/download/TL-MR6400.html#Firmware

Exact same problem here. I have a Cisco 877 router connected to port LAN1 on the MR6400, it initiates a site to site VPN to another Cisco router across the internet with traffic passing through the...