Hello again everyone. I wanted to touch base with you this week and let you know how things are going on our end. To provide some clarity, there were a couple of formal investigations opened related to this issue. The first one was related to invoice delivery status showing incorrectly, the other was about email delivery rate. 

As mentioned by @T3chNoid and @JD93, QBO is now correctly showing the status of invoices sent to customers so you know when an email did or did not get delivered. On the latter, our team has deployed some changes which have improved the delivery of emails and continue to work on further improving it.

I'll be back here to provide more updates as they are available.