So this is just a continuation of this issue: HELP: LAN IP is the same as IPv4
Hence there is still another DHCP server active in your network which distributes an invalid subnet with a default route which points to nowhere.
So a static IP without DHCP client is still the solution, and anyway reasonable for a Pi-hole DNS resolver.
I guess
pihole-FTL dhcp-discover
does not trigger any DHCP offer from that 192.168.227.1 rogue server, i.e. it is not visible in the tricorder upload of this issue either, right?
Does anyone have a good idea how to detect a DHCP server which distributes a default route that does not exist? Not visible on the true router either, since it is in that different subnet. tcpdump could work, e.g. on a detachable shell session, so the command can just run for a day or so and does not abort if the SSH connection does:
apt install tcpdump
tcpdump -i eth0 udp port 68
EDIT: Or a running a network scanner with ping (or something on port 67 which DHCP servers respond to) through 192.168.227.1 - 192.168.227.254 range, as that DHCP server should have an IP in that range as well. Why not just that:
for i in {1..254}; do ping -c1 -W1 -q "192.168.227.$i"; done
A attempt and 1 second timeout per IP, could be tweaked, but should usually work for hosts in same LAN which do respond to pings at all.