By default, Raspberry Pi OS generates stable IPv6 interface identifiers according to RFC7217 (instead of EUI64 ones that would contain and disclose a NICs MAC). They serve a similar purpose as RFC4941 IPv6 Privacy Extension interface ids, but unlike those, they are calculated by your OS only once and are generally designed to change only if the IPv6 prefix would change.
Now, it seems your OS update would have triggered to calculate those interface identifiers afresh:
*** [ DIAGNOSING ]: Network interfaces and addresses
2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 inet6 fe80::17ac:6b39:36c6:70f5/64 scope link
Likely, you are observing your network still sending DNS requests to the previous IPv6.
To address this, just update your Pi-hole's IPv6 in your router.