Unbound DNS keeps on returning SERVFAIL status

Seems like you are indeed affected by WARNING: Raspbian October 2021 release bullseye + unbound.

To fix this, 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