I was wrong about that IPv6 address beloging to your router:
Those are Charter's / Spectrum's IPv6 DNS servers (rns01.charter.com).
Your router is offering them via IPv6, which also means any other device on your network may pick them up and bypass Pi-hole.
You may be able to override those on your RPi (e.g. by adding Pi-hole's IPv6 localhost address ::1 to domain_name_servers), but you should really configure your router to offer Pi-hole's IPv6 address instead.