For diagnosing, configure something else in Pi-hole for upstream DNS resolution (not unbound).
You can do below to output verbose debug logging for unbound to screen (dont need to change port etc):
sudo service unbound stop
sudo /usr/sbin/unbound -ddd -vvv -c /etc/unbound/unbound.conf
Inspect for errors/warnings when started and see whats logged when you do below dig in another SSH session:
dig @localhost -p 5335 a www.instagram.com.
EDIT: Might want to compare with a dig for another domain that you know works.
When finished, you can break with CTRL-C and startup unbound again via init/systemd:
sudo service unbound start
And configure Pi-hole again to use unbound upstream when finished diagnosing.
Did you read below mentioned thread/link and tried some of those commands/queries for your troubled SERVFAIL domain?