All deleted invoices would go under the ‘deleted’ option at the foot of the invoice management screen. If you could supply me with an example, I’m happy to take a closer look for you? You’re welcome to send the details to me in a private message (click my name and then ‘Message’).
Invoice numbers should be continuous. When you create an invoice, it will pick up the next available number/letter in the sequence. All active (used) invoice numbers will be either under ‘Active’ or ‘Archived’: