With IPv4 just about every consumer and ISP supplied router will use private addresses. This means that you have NAT protecting you by default and you have to purposefully port forward to open a hole in your network.
IPv6 uses public addresses (most of the time) and each computer/server/router/device/camera/thermostat/etc... will have a directly accessible IPv6 address. Everything with that public address space is basically sitting on the internet without anything in between. Public IPv6 prefixes are not for someone that doesn't have full knowledge of what/why/how it works.
Granted ISPs (mostly in Germany from my experience) rotate that address space and give you a new address, but that's just shuffling cards, your computers and electronics are still fully accessible.
For proof, go to your Pi-hole admin page and look for a query to gum.drop that just came from me.
Remove the IPv6 address from /etc/pihole/setupVars.conf since it doesn't exist anymore.
Uncheck the Use IPv6 box in the DHCP server configuration page, no need for that now.
Edit: And for good measure you can run pihole -r and reconfigure, skip the IPv6 setup if it's detected. There's a few places the old IPv6 address is still seen but it's not actually enabled on the Pi WLAN0.
