Have the same problem, just noticed that it send to the last email as I send the invoice to the client and a copy to our own department which requested the invoice.
Sometimes a family request an invoice to be sent to an organisation and now I noticed the family received the reminder instead of the organisation whom is paying.
So the last person send doesn't have to be the person paying, but the email address on the card is paying. I have received complaints due to this.
It is maybe how MOYB thinks it should work but practical it doesn't.