DNS not reachable via UDP/Port 53

We have recently added an external DNS lookup server address in our Link controller under the System --> configuration --> device --> dns --> DNS Lookup Server List

However, when we do a normal nslookup to any url (e.g: google.com), it is unable to resolve the DNS.

    [Active:In Sync] config  dig @8.8.8.8 google.com

    ; <<>> DiG 9.9.6-P1 <<>> @8.8.8.8 google.com
    ; (1 server found)
    ;; global options: +cmd
    ;; connection timed out; no servers could be reached

    [root@23ZLC02:Active:In Sync] config  nslookup google.com
     ;; connection timed out; no servers could be reached

But LC has external connectivity:

[Active:In Sync] config  ping 8.8.8.8
PING 8.8.8.8 (8.8.8.8) 56(84) bytes of data.
64 bytes from 8.8.8.8: icmp_seq=1 ttl=45 time=3.23 ms
64 bytes from 8.8.8.8: icmp_seq=2 ttl=45 time=3.16 ms
64 bytes from 8.8.8.8: icmp_seq=3 ttl=45 time=4.32 ms
64 bytes from 8.8.8.8: icmp_seq=4 ttl=45 time=7.91 ms
64 bytes from 8.8.8.8: icmp_seq=5 ttl=45 time=3.30 ms

However, it does work if we use the dig command with tcp:

[Active:In Sync] config  dig @8.8.8.8 google.com +tcp

; <<>> DiG 9.9.6-P1 <<>> @8.8.8.8 google.com +tcp
; (1 server found)
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 47990
;; flags: qr rd ra; QUERY: 1, ANSWER: 6, AUTHORITY: 0, ADDITIONAL: 1

;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 512
;; QUESTION SECTION:
;google.com.                    IN      A

;; ANSWER SECTION:
google.com.             299     IN      A       74.125.24.100
google.com.             299     IN      A       74.125.24.139
google.com.             299     IN      A       74.125.24.138
google.com.             299     IN      A       74.125.24.101
google.com.             299     IN      A       74.125.24.102
google.com.             299     IN      A       74.125.24.113

;; Query time: 9 msec
;; SERVER: 8.8.8.853(8.8.8.8)
;; WHEN: Tue Jan 09 21:17:10 SGT 2018
;; MSG SIZE  rcvd: 135

Here is the TCPDUMP output:

[Active:In Sync] config  tcpdump -ni 0.0 -s0 host 8.8.8.8 and udp port 53
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on 0.0, link-type EN10MB (Ethernet), capture size 65535 bytes
19:34:58.297492 IP 203.127.17.5.54771 > 8.8.8.8.domain:  9669+ [1au] A? google.com. (39) out slot1/tmm0 lis=
19:35:03.297323 IP 203.127.17.5.54771 > 8.8.8.8.domain:  9669+ [1au] A? google.com. (39) out slot1/tmm0 lis=
19:35:08.297432 IP 203.127.17.5.54771 > 8.8.8.8.domain:  9669+ [1au] A? google.com. (39) out slot1/tmm0 lis=

Port 53 seems to be open as well:

[Active:In Sync] config  telnet 8.8.8.8 53
Trying 8.8.8.8...
Connected to 8.8.8.8.
Escape character is '^]'.
Connection closed by foreign host.

The network connection flow is Link controller --> IPS --> Router --> Internet