Therefore you have to distribute pihole's IP as DNS server via DHCP. This has to be configured at your router. Once you did, you have to de/reconnect your clients to the network to pickup the new settings.
This indicates a network configuration erorr.
Depending on what kind of IPv6 addresses that are your router might assign global IPv6 addresss to your devices. They could use them to circumvent pihole and query another DNS server. Do you need IPv6? If not, turn it of in your router or use ULA