Since the voided invoice is an unpaid invoice, it sends that email template. You can update the email template if you'd like to be more specific. For example, since that email template accepts multiple invoices, you might check that the status of the first invoice and set the subject conditionally:
{% if invoices[0].status == "void" %}Invoice Voided{% else %}Invoice Due{% endif %}
I did not test this, so you should before using it.