@velociraptor thanks
Like the OP, I could only reach my pi-hole by just the hostname (from /etc/hostname), or pi.hole. I wanted to address the pihole with hostname.lan where "lan" is the domain configured in the router.
I added this to /etc/dnsmasq.d/01-pihole.conf
domain=lan
local=/lan/
The local setting fixed another problem which was that pi-hole was forwarding many ***.lan to the external DNS server. Perfect.