Hello,
Our QA team have managed to reproduce the issue in our environment. I'll update this topic as soon as we fix the issue.
For now, to create Exchange mailboxes automatically, you can use the following PowerShell script:
Add-PSSnapin Microsoft.Exchange.Management.PowerShell.Admin
$database = "Mail01\Database01"
Enable-Mailbox -Identity "%distinguishedName%" -Alias "%username%" -Database $database