Apex domains cannot have CNAME records.
Question and response
Question
dig u-v1.mx.microsoft. NS
Answer
u-v1.mx.microsoft. 5 NS ns2-86.azure-dns.net.
u-v1.mx.microsoft. 5 NS ns4-86.azure-dns.info.
u-v1.mx.microsoft. 5 NS ns1-86.azure-dns.com.
u-v1.mx.microsoft. 5 NS ns3-86.azure-dns.org.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0