Your router is distributing another DNS server along with Pi-hole's IP, allowing clients to bypass Pi-hole via 1.1.1.1:
*** [ DIAGNOSING ]: Discovering active DHCP servers (takes 10 seconds)
Scanning all your interfaces for DHCP servers
* Received 318 bytes from eth0:192.168.1.1
Server IP address: 192.168.1.1
DHCP options:
Message type: DHCPOFFER (2)
dns-server: 192.168.1.20
dns-server: 1.1.1.1
The extra DNS server is gone, but there's not a many clients using your Pi-hole yet:
*** [ DIAGNOSING ]: contents of /var/log
-rw-r--r-- 1 pihole pihole 18762 Feb 25 22:10 /var/log/pihole-FTL.log
-----tail of pihole-FTL.log------
[2021-02-25 23:10:41.208 508M] Imported 864 queries from the long-term database
[2021-02-25 23:10:41.208 508M] -> Total DNS queries: 864
[2021-02-25 23:10:41.208 508M] -> Cached DNS queries: 7
[2021-02-25 23:10:41.208 508M] -> Forwarded DNS queries: 819
[2021-02-25 23:10:41.208 508M] -> Blocked DNS queries: 38
[2021-02-25 23:10:41.208 508M] -> Unknown DNS queries: 0
[2021-02-25 23:10:41.208 508M] -> Unique domains: 61
[2021-02-25 23:10:41.208 508M] -> Unique clients: 3
[2021-02-25 23:10:41.208 508M] -> Known forward destinations: 1
[2021-02-25 23:10:41.208 508M] Successfully accessed setupVars.conf
Did your clients renew their leases yet?
Your clients will dismiss the extra DNS server only upon acquiring a new DHCP lease. You may force a client to do so, e.g. by dis- and reconnecting WiFi or by poeer-cycling it.