Okay, just got off the phone with my ISP.
Turns out they don't know how I got a connection in the first place 
.
According to them, no client of them within the country has IPv6 enabled on residential connections, and they advised me to simply ignore it, so at this point I'm just plain confused.
Clearly something is connected, given I'm able to perform digs, but at least now I know it's an ISP problem and not a Pi-Hole problem.
Anyway, thanks for the help everyone! Looks like disabling IPv6 within my router will be an easier fix for the moment.