I have a similar problem specifically with DNS and IPv6: my ISP (UPC in Poland) does not allow to change the DNS given for IPv6.

So when a device in my network connects it gets an IPv4 and the pihole DNS for IPv4, but also gets an IPv6 and my ISP's DNS for IPv6.

As I've noticed, all OSes prefer resolving names over IPv6, making the pihole useless.

The only solutions so far are disabling IPv6 which is a shame or setting DNS manually on each device which is impossible on Android.