That domain is resolving correctly through my unbound instance (click for details):
$ dig github.com @127.0.1.1 -p 5335
; <<>> DiG 9.11.5-P4-5.1+deb10u3-Debian <<>> github.com @127.0.1.1 -p 5335
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 14069
;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 1
;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 1472
;; QUESTION SECTION:
;github.com. IN A
;; ANSWER SECTION:
github.com. 60 IN A 140.82.121.3
;; Query time: 41 msec
;; SERVER: 127.0.1.1#5335(127.0.1.1)
;; WHEN: Wed May 12 08:54:31 CEST 2021
;; MSG SIZE rcvd: 55
It is not uncommon to observe SERVFAILs every now and then, see also my older, more detailed reply in MS Teams gets no presence status for contacts - #6 by Bucking_Horn.