There is little reason for IPv6 prefix to not be a static assignment, but I do remember when I was using it on sky, and they dynamically allocated prefixes.
Assuming its a static allocation there is ways to utilise it without relying on dynamic services, as an example on my own configuration, my prefix from AAISP is configured statically on my LAN interfaces on my firewall, which is then able to dynamically allocate to clients as needed. I only use the DHCP6 service to get a /128 for the firewall's own WAN service, but as I discovered recently if that /128 isnt there, it just falls back to the static configuration for its own direct access.
This way even if there is a dhcp/slaac issue it would still stay online.