How to resolve Unbound causing N/A Reply for certain domains?

Anybody running Unbound (and Pi-Hole) on their RPi?

I've been using Pi-Hole on my RPi4 for several years now and a couple
months ago I decided to give Unbound a go. I followed the instructions
as described in https://docs.pi-hole.net/guides/dns/unbound/ and set my
DNS in Pi-Hole to 127.0.0.1#5335 (DNSSEC is unchecked). Everything seems
to work fine, but then I noticed certain domains result in Server Not
Found and the Query Log shows an N/A Reply for these sites.

These are some examples:

    https://economie.fgov.be/ (in fact everything *.fgov.be)

    https://argenta.be/

    https://www.paypal-opladen.be/

The first is a government site, the second a bank and the third a site
to charge a paypal account. I don't know why, I'm not an expert, but
I've got the impression it's not Unbound that is to blame, but the sites
themselves (DNSSEC?). I've searched and searched for the N/A problem,
but found no solution.

A Reddit user with Pi-Hole and Unbound could reach these sites without a
problem. I've started with a fresh image (with Pi-Hole installed) and
reinstalled Unbound, but the problem persists.

Pi-Hole is running under Bookworm (latest updates)
Pi-hole v5.18.4 FTL v5.25.2 Web Interface v5.21

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