Run the below command against each mailbox to see the value for IsExcludedFromProvisioning

• Get-MailboxDatabase -Server <ServerName>| select Name,ServerName,IsExcludedFromProvisioning

By default, all the database have False value on in the IsExcludedFromProvisioning

The databases you want excluded from automatic provisioning of new mailboxes; set the value of IsExcludedProvisioning to $true

• Set-MailboxDatabase "Database Name" -IsExcludedFromProvisioning $true