Public TXT record for internal zone

Your test dig @ns0.transip.net dig _acme-challenge.vanzanten.be is wrong in two ways:

• There is a stray dig in the middle, causing weirdness.
• The type of the expected record is not A, but TXT.

dig @ns0.transip.net _acme-challenge.vanzanten.be TXT would be the appropriate test, as per:

$ dig @ns0.transip.net _acme-challenge.vanzanten.be TXT +norec

; <<>> DiG 9.11.25-RedHat-9.11.25-2.fc33 <<>> @ns0.transip.net _acme-challenge.vanzanten.be TXT +norec
; (2 servers found)
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 56046
;; flags: qr aa; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 1

;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 4096
;; QUESTION SECTION:
;_acme-challenge.vanzanten.be.  IN      TXT

;; ANSWER SECTION:
_acme-challenge.vanzanten.be. 300 IN    TXT     "hT1NqF_CMjuQnNagi_dvnyG7WSRKmda-AUBt6ELKbyY"

;; Query time: 7 msec
;; SERVER: 2a01:7c8:dddd:195::195#53(2a01:7c8:dddd:195::195)
;; WHEN: Sun Dec 27 20:24:07 UTC 2020
;; MSG SIZE  rcvd: 141

$