Finally found it how to resolve after checking the code itself, phew!
You to go to Billing Overview(each staff account), scroll all the way down to Orders, click the setting icon, and you see the Notifications setting!
You can select always send for Email.
If u want to see the code or db, the table to reference is order_staff_settings, you can search the code or db.
But I also found some reply few yrs ago.. might be a good reference.
Hope this helps someone!