New subdomain but e-mail addresses are already in use?

I just created a new subdomain in our M365 for special purpose e-mails & 3rd party mailing support. When I attempt to create a new shared mailbox mailto:address@subdomain.domain.tld I get the error that the e-mail address is already in. The group in question, mailto:address@domain.tld, is an AD synced distribution group and most certainly is NOT using the subdomain in any aliases. I've checked EOL powershell get-recipient and there is no recipient for mailto:address@subdomainand it's not associated with any SharePoint sites or Teams.

In M365 Admin Center, when I attempt to create the shared mailbox, I get the error that it is in use by the group "address", which the admin center calls a "null team", but is the AD synced DG by its primary e-mail address.

In EOL EAC if I attempt to create the shared mailbox there, it just says the "The email address is already being used."

This is a brand new, never been heard of, never been seen before subdomain. Created in M365 as a new domain (cause it's easier than trying to add subdomain support in EAC and easier to manage). What gives?