Hmm.. this is where we are in disagreement.
Here is the snippet from my /etc/dhcpcd.conf file. This is what the portion of the install script did to my configuration.
interface eth0
static ip_address=192.168.0.8/24
static routers=192.168.0.1
static domain_name_servers=149.112.121.30 149.112.122.30
Previously - my DHCP configuration would have (pre-pi-hole) given the router (192.168.0.1) as the DNS server on my network.
The two addresses in static domain_name_servers=149.112.121.30 149.112.122.30 are the two custom DNS servers I entered when I ran the setup script.
I'm claiming there is a bug here in the script.
You are only asked once to specify the upstream DNS servers to use for pi-hole. At least when you specify a custom set of servers, the script is re-using those same addresses for the static configuration section.
Hmm.. I'm now doubting myself - maybe I'll setup a VM and test this out again, but reading the script - it really looks like this is wrong for the scenario I call out. PIHOLE_DNS_1 and PIHOLE_DNS_2 are used for both the upstream pi-hole server and for the static configuration (if needed).