Per this doc:
While we strongly recommend against this network configuration (see article), using Azure AD Connect sync with a single label domain is supported, as long as the network configuration for the single level domain is functioning correctly. In SLD scenarios where the Active Directory NetBIOS domain name differs from the FQDN domain name, it's unsupported to install Azure AD Connect.
Is that the case? Does the netbios match the single label domain?
Microsoft does not recommend this config of course:
https://learn.microsoft.com/en-US/troubleshoot/windows-server/networking/single-label-domains-support-policy
If you cant get it to work, I would open a support case with Azure support