Immutable SMTP aliases in Exchange Online

In researching why we couldn’t remove a *@[PRIMARY TENANT DOMAIN].onmicrosoft.com SMTP alias as part of an account re-name clean-up we discovered and confirmed with MSFT support that upon creation of a mail object in Exchange online, a permanent address of  <alias>@<primary_tenant_domain>.onmicrosoft.com is given to the object. Such that even when the account is renamed and its alias changes , this immutable address will remain on that renamed object. Even if a new mail object is created with a name\alias matching a previous object that was re-named, it would result in an immutable address of <alias>1@<primary_tenant_domain>.onmicrosoft.com on the new object.