Above is the rogue config file that doesnt belong to Pi-hole.
Change listening behaviour for the pihole-FTL daemon from "all" into "single" with below:
pihole -a -i single
If you run below one again, you'll see the interface= directive shows up twice now with both interfaces:
(EDIT: Its a sort of hack but it means you dont have to delete that rogue config file)
sudo rgrep -v '^ *#\|^$' /etc/dnsmasq.*
FYI (local is the default):
$ pihole -a -i -h
Usage: pihole -a -i [interface]
Example: 'pihole -a -i local'
Specify dnsmasq's network interface listening behavior
Interfaces:
local Only respond to queries from devices that
are at most one hop away (local devices)
single Respond only on interface eth0
bind Bind only on interface eth0
all Listen on all interfaces, permit all origins