It might be worth doing a packet capture on your WAN, you can stream it straight to wireshark's udpdump, and look if there's any sort of reply to DHCP or any RA broadcasts, you can just use ipv6 as a filter.
Also come to think of it, DHCPv6 is usually still used for your prefix so that should work regardless of SLAAC. SLAAC is often just used for your default routes and the router's own IP (as allocating that from your prefix manually is often considered a misconfiguration).