I think you'll need to inspect the Unbound logs to try and figure out whats wrong.
For below instructions, I'm assuming you dont have Unbound configured as upstream for Pi-hole yet!
Up above verbosity to three first to have individual client queries logged in the systemd journals.
One-liner:
sudo sed -i 's/verbosity: 0/verbosity: 3/' /etc/unbound/unbound.conf.d/pi-hole.conf
Follow/tail the systemd journal live for the Unbound unit with below:
sudo journalctl --full --no-hostname --follow --lines 0 --unit unbound.service
Open another shell/SSH session and post the appearing log entries here after you've restarted Unbound with below:
sudo systemctl restart unbound.service
And separately, post also the log entries when performing below query for an "any site":
dig +short @127.0.0.1 -p 5335 pi-hole.net
When done diagnosing, run below two to restore verbosity to zero again:
sudo sed -i 's/verbosity: 3/verbosity: 0/' /etc/unbound/unbound.conf.d/pi-hole.conf
sudo systemctl reload unbound.service
FYI:
$ man unbound.conf
[..]
verbosity: <number>
The verbosity number, level 0 means no verbosity, only er‐
rors. Level 1 gives operational information. Level 2 gives
detailed operational information including short information
per query. Level 3 gives query level information, output
per query. Level 4 gives algorithm level information.
Level 5 logs client identification for cache misses. De‐
fault is level 1. The verbosity can also be increased from
the commandline, see unbound(8).