I found a solution/workaround. I had a similar issue. Sent a voting email to multiple recipients and only one response showed in tracking despite receiving several in my inbox.
The answer? I had only opened/viewed one. The rest I had marked as read.
Once I opened all the messages individually, they all showed up in tracking.
This is reasonable for small distributions, but large ones could be time-consuming. It’s not the tracking behavior I was expecting. 🤷♀️
Anyways, I wanted to pass this along. Hope it helps someone.