I feel this could be due to the expiration of "Security Association". If there is no user traffic for a duration longer than the SA lifetime, the SA will expire and the new SA will be formed only when there is a user traffic initiated. Look at the logs for more information on why the VPN is breaking.
You can try configuring "keepalives" between the VPN peers so that the VPN is always on and does not timeout.
We have configured the outside and inside Interface with official ipv6 adresses, set a default route on outside Interface to our router, we also have definied a rule , which also gets hits, to permit tcp from inside Interface to any6.
In Syslog I also se...