I have a pretty vanilla router configuration on an older Kickstarter Omnia, which I just reflashed with the latest medkit. I recently switched to a new provider with IPv6 support. With ds-lite disabled for wan, the interface gets a proper (though dynamic) IPv4 and an IPv6 through DHCP. It also gets a /56 IPv6 prefix, of which I delegate a /64 prefix to br-lan and another /64 to br-guest-turris. Network devices on LAN get multiple IPv6 addresses from the router through DHCP and SLAAC, among which also global unicast addresses from the delegated prefix.
Pinging the router from an external source works initially to both the /128 IPv6 address of the wan6 interface and the ::1/64 address on br-lan. Pinging LAN devices also works. Ping -6 google.com from LAN also works.
The problem: after a few hours IPv6 routing between WAN and LAN quits working. I can still ping the router from outside on its /128 WAN address, but none of the /56 delegated prefix addresses work anymore. Ping -6 google.com from LAN also fails. In the mean time IPv4 NAT and port forwards work. /etc/init.d/network restart quickly solves the issue for a few hours.
How do I go about debugging this?