Cannot resolve sp.edu.sg

Zero Trust 1.1.1.1
1 
REDACTED]~]$ dig @1.1.1.1 sp.edu.sg

        ;<<>> DiG 9.10.6 <<>> @1.1.1.1 sp.edu.sg
  ;(1 server found)
  ;;global options: +cmd
  ;;Got answer:
  ;;->>HEADER<<- opcode: QUERY, status: SERVFAIL, id: 23824
  ;;flags: qr rd ra      ;QUERY: 1, ANSWER: 0, AUTHORITY: 0, ADDITIONAL: 1

  ;;OPT PSEUDOSECTION:
  ;EDNS: version: 0, flags:      ;udp: 1452
  ;;QUESTION SECTION:
;    sp.edu.sg.			IN	A

  ;;Query time: 146 msec
  ;;SERVER: 1.1.1.1#53(1.1.1.1)
  ;;WHEN: Wed Oct 03 21:48:37 +08 2018
  ;;MSG SIZE  rcvd: 38

REDACTED]~]$ dig @1.0.0.1 sp.edu.sg

        ;<<>> DiG 9.10.6 <<>> @1.0.0.1 sp.edu.sg
  ;(1 server found)
  ;;global options: +cmd
  ;;Got answer:
  ;;->>HEADER<<- opcode: QUERY, status: SERVFAIL, id: 49900
  ;;flags: qr rd ra      ;QUERY: 1, ANSWER: 0, AUTHORITY: 0, ADDITIONAL: 1

  ;;OPT PSEUDOSECTION:
  ;EDNS: version: 0, flags:      ;udp: 1452
  ;;QUESTION SECTION:
;    sp.edu.sg.			IN	A

  ;;Query time: 150 msec
  ;;SERVER: 1.0.0.1#53(1.0.0.1)
  ;;WHEN: Wed Oct 03 21:49:16 +08 2018
  ;;MSG SIZE  rcvd: 38

REDACTED]~]$ dig @8.8.8.8 sp.edu.sg

        ;<<>> DiG 9.10.6 <<>> @8.8.8.8 sp.edu.sg
  ;(1 server found)
  ;;global options: +cmd
  ;;Got answer:
  ;;->>HEADER<<- opcode: QUERY, status NOERROR, id: 17844
  ;;flags: qr rd ra ad      ;QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 1

  ;;OPT PSEUDOSECTION:
  ;EDNS: version: 0, flags:      ;udp: 512
  ;;QUESTION SECTION:
;    sp.edu.sg.			IN	A

  ;;ANSWER SECTION:
sp.edu.sg.		2013	IN	A	35.201.83.130

  ;;Query time: 43 msec
  ;;SERVER: 8.8.8.8#53(8.8.8.8)
  ;;WHEN: Wed Oct 03 21:50:03 +08 2018
  ;;MSG SIZE  rcvd: 54

REDACTED]~]$ dig +short CHAOS TXT id.server @1.1.1.1
"SIN"

REDACTED]~]$ dig +short CHAOS TXT id.server @1.0.0.1
"SIN"

dnzviz.net result: http://dnsviz.net/d/sp.edu.sg/dnssec/