It is not possible to add a custom dns CNAME entry with content @, which points back to the zone name (domain.tld or subdomain.domain.tld).
When a user tries this the zone becomes invalid and manual removal is required.
Reproduce:
Login as a user.
Create custom dns record
choose CNAME, give the record a name and fill in @ for content.
Wait a few seconds and check out the domain status.