It is the same story as with other thread for updating to PiHole 6 while the OS was still on v10. I have since setup a new SD with RasPi OS v12 and all is working fine.
I have seen comments on the web suggesting not to do an RPi stepped OS version upgrade from Debian v10 to v12 on the same SD but rather use a new one for a fresh installation. As I had a spare one, I opted doing it this way.