Clients can only access internet with DNS manually set to PiHole

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Before altering any router settings, first check if Pi-hole responds via IPv4 by running below two on a Windows, MacOS or Linux client (with 10.0.0.2 being my Pi-hole instance):

C:\>nslookup pi.hole 10.0.0.2
Server:  ph6a.home.dehakkelaar.nl
Address:  10.0.0.2

Name:    pi.hole.home.dehakkelaar.nl
Addresses:  fe80::XXX
          10.0.0.2

C:\>nslookup pi-hole.net 10.0.0.2
Server:  ph6a.home.dehakkelaar.nl
Address:  10.0.0.2

Non-authoritative answer:
Name:    pi-hole.net
Address:  162.244.93.14

If that works, next you have to make sure your router doesnt advertise any IPv6 DNS servers to the clients via IPv6 RA (Router Advertisement) or via DHCPv6 that would allow the clients to bypass Pi-hole.
DHCPv6 should be disabled anyway on the router bc it isnt supported on a bunch of OS'es.
You can check for any RDNSS (Recursive DNS Server) addresses advertised via IPv6 RA if run below on the Pi.
Below will broadcast an IPv4 DHCPDISCOVER plus an IPv6 RA solicitation and catches the response from the router(s):

sudo pihole-FTL dhcp-discover

Dont post unredacted output for above one here for privacy!
Below an example of IPv6 RDNSS addresses advertised via IPv6 RA:

EDIT: Ps, is below fixed now?