There has been a recent (around 24th June) change, I suspect to netifd, that has changed the ip6class assigned to an interface and in my case stopped ISP prefix delegation to my LAN interface.
For 'reasons' I prefer not to delegate ULA prefix to my LAN and to do this I previously used
list ip6class 'wan6' as an option in /etc/config/network to filter out all prefixes except the prefix assigned to 'wan6'.
After the June 24 change, I got zero IPv6 prefixes allocated to my LAN interface. It took me a long time to work out that the 'ip6class' value from the 'wan6' interface had changed.
Luci reported the value as 'a1a5b3d2 (wan6)' and updated the stored value in /etc/config/network to 'a1a5b3d2'
eg:
list ip6class 'a1a5b3d2'
I'm posting this here in case it helps someone else solve a prefix delegation mystery.