I have Pi-Hole running with Unbound as upstream resolver.
In the past after installing Unbound I did these adjustments to Pi-Hole:
Open the configuration file /etc/dnsmasq.d/01-pihole.conf and make sure that cache size is zero by setting cache-size=0. This step is important because the caching is already handled by the Unbound Please note that the changes made to this file will be overwritten once you update/modify Pi-hole.
When you're using unbound you're relying on that for DNSSEC validation and caching, and pi-hole doing those same things are just going to waste time validating DNSSEC twice. In order to resolve this issue you need to untick the Use DNSSEC option in Pi-hole web interface by navigating to Settings > DNS > Advanced DNS settings.
Is this till the case? Since Unbound en Pi-Hole have been updated.