The client pi.hole triggers a warning in dnsmasq core that the maximum number of concurrent DNS requests has been reached

You may be affected by a DNS loop created between your upstream unbound and Pi-hole. For a complete background, see WARNING: Raspbian October 2021 release bullseye + unbound.

Please try:

  1. Edit file /etc/resolvconf.conf and comment out the last line which should read:

unbound_conf=/etc/unbound/unbound.conf.d/resolvconf_resolvers.conf

  1. Delete the unwanted unbound configuration file:

sudo rm /etc/unbound/unbound.conf.d/resolvconf_resolvers.conf

  1. Restart unbound:

sudo service unbound restart