Apex domains cannot have CNAME records.
Question and response
Question
dig dub1.discourse.cloud. NS
Answer
dub1.discourse.cloud. 2 NS ns1b.dub1.discourse.cloud.
dub1.discourse.cloud. 2 NS ns2.discourse.cloud.
dub1.discourse.cloud. 2 NS ns3.discourse.cloud.
dub1.discourse.cloud. 2 NS ns1a.dub1.discourse.cloud.
dub1.discourse.cloud. 2 NS ns1.discourse.cloud.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0