Survey invite hard bounce, no reports and no CX dashboard reports to specific email addresses

That sounds similar to something my organization has been going through for the past year or so. Did Qualtrics Support confirm that your email address and the no reply address had not been flagged on their end as undeliverable? If they have not checked, I would ask them to do so.
What's happened is that my organization's email security solution and Qualtrics (to put it in simpleton terms for someone like me) do not communicate very well with each other, which every so often causes one or more employee email addresses to be flagged as undeliverable by Qualtrics, even though Qualtrics IP address ranges are allowlisted. I discovered this using Workflow emails to send notifications to employees. The emails are flagged seemingly at random; there is no pattern or sequence of events that leads up to it occurring.
This may or may not be the same thing you're going through, but it's worth ruling out. Unfortunately no one has come up with a viable solution, and I've had to abandon using Workflow emails to send notifications to co-workers for my larger projects. The only "solution" is to take action after the recipient happens to notice they've stopped receiving emails, and contact Qualtrics Support to request that the email(s) be unflagged.
Our email security solution says they have done everything thing they can to enable delivery of messages from Qualtrics, and that there is no evidence of too many messages being sent to an employee or repeated bounces (as Qualtrics Support suggests was happening). Qualtrics, to be blunt, doesn't seem to believe there could be anything wrong on their end. I've asked them to look into the problem further, requested to receive a notification anytime an address in my organization's domain is flagged, and even submitted a product idea, but have made little progress in addressing the problem.
All that said, I hope that this is not the same issue you're going through, and that yours is an easier fix!

Hi MatthewM ,
Just wanted to provide an update and for anyone else experiencing this problem. I think you were right. They had blocked my email address as I had exceeded the limit (what that limit is though I don't know). This is the message I got from the support person I was in contact with:
"For this specific case the reason was listed as "unknown" on our end but when Qualtrics has tried to email a particular person too many times and the email repeatedly bounces, the exact same email is sent multiple times [I place a hidden timestamp on many of my emails to prevent this so I don't think this was the culprit], or repeated emails are never opened, the system makes a note of the email and stops sending them emails from our servers. This is to maintain a good sender score (which works like a credit score for an IP address and depends on how many of our emails bounce as well as other factors). Sender score is important because some email domains will reject messages from us if our sender score is too low"
So, there we go. The tip was to keep pushing and pushing the support team, telling them that the problem was at their end and finally they looked in the right place. I personally think that this issue has not been handled at all well. If they don't provide us with the correct access or reporting they need faster and better response for issues like this for admins, for by their very nature they are cc'd into most surveys and reports.

I have asked them to do a root cause as I got past from post to post on this which inevitably delayed the resolution so I hope in future, other clients experiencing this will not have to wait for such a long time compared to what I have. That said - we're still stuck with limited reporting especially on the CX side - it's like a blackhole! lol

Anyway, thanks for your help on this and lets hope this doesn't happen again. :s

Rod Pestell