invoice for canceled orders

Invoices can be created either automatically or manually.
Automatically only happens via email attachment (assuming you are using the free version of this plugin with no additional code to generate the invoice). If you attach the invoice only to the ‘completed’ order email, it will not be generated until that email is sent (e.g. order is completed).
However, if you also attach the invoice to the ‘New order” admin email, the invoice will be generated as soon as the order is placed and you (as admin) receive the email + invoice.
Additionally, if an order would go to completed first and then is cancelled, the invoice will already have been generated.

Additionally, the invoice can be generated manually, independent of the order status – for example by the bulk actions (select orders + and apply the ‘PDF Invoices” action) or by clicking the button in the actions column or on the order details page. This button will always show (invoice or not), but you can see if an invoice has been created by the green check mark on the button.

I suspect that you may have attached the invoice to the admin new order email, is that possible?

You could prevent the creation of invoices (automatically or manually) via the ‘disable for’ setting under Documents > Invoice, as an extra safety measure. This would also prevent the invoice from being attached to the admin email if the order is not completed yet.