This probably isn't very important, but: the nameservers include additional records for ns1-4, but not ns5:
Code:
$ dig test2.slacknode.com.test ns @ns1.linode.com
...
;; ANSWER SECTION:
test2.slacknode.com.test. 86400 IN NS ns5.linode.com.
test2.slacknode.com.test. 86400 IN NS ns1.linode.com.
test2.slacknode.com.test. 86400 IN NS ns3.linode.com.
test2.slacknode.com.test. 86400 IN NS ns4.linode.com.
test2.slacknode.com.test. 86400 IN NS ns2.linode.com.
;; ADDITIONAL SECTION:
ns1.linode.com. 86400 IN A 69.93.127.10
ns2.linode.com. 86400 IN A 65.19.178.10
ns3.linode.com. 86400 IN A 75.127.96.10
ns4.linode.com. 86400 IN A 207.192.70.10
...
It's the same on all of them, including ns5.