Apex domains cannot have CNAME records.
Question and response
Question
dig g1a2ac88.usw1.send.mailgun.net. NS
Answer
g1a2ac88.usw1.send.mailgun.net. 5 NS ns-cloud-b4.googledomains.com.
g1a2ac88.usw1.send.mailgun.net. 5 NS ns-cloud-b2.googledomains.com.
g1a2ac88.usw1.send.mailgun.net. 5 NS ns-cloud-b1.googledomains.com.
g1a2ac88.usw1.send.mailgun.net. 5 NS ns-cloud-b3.googledomains.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0