Hello. For a long time (almost a year) on different software versions we have one problem: 6to4 tunnel stops transfer traffic. It works right after creation, works for a week, may be months, but in the end it dies. There is no Rx data on 6to4 interface on both sides. The only possible solution - delete 6to4 interfaces on both sides, reboot both routers and create interfaces again.
Please tell me how to get developers attention for this issue?
Right now I again “fix” my problem and updated to 6.40.1 version. One end is RB1100AHx2, other end is RB2011UAS-2HnD.
P.S. I’m in Riga, Gertrudes street 
Maybe you should invite the developers for a beer?
Temporary workaround:
set netwatch script to monitor remote ipv6 address. If address down, execute code: /ip firewall connection { remove [find protocol=ipv6] }
Did you allow incoming traffic for that protocol? (at least with source address of your peer)