When reviewing an Acknowledgment Email report, you see a status of Queued next to an email address or multiple email addresses. You may also see an Opened Date or Opened Date may be missing.
When we see a combination of Queued with Opened, the Opened indicates the message was indeed sent and Opened; as we don't see Queued Updated to Sent, it means there was likely a temporary interruption in the process and the status wasn't updated. If it's only been a few minutes, it's likely the status will update. If it's been a while, let's say a day or so, the status likely will be stuck in its current state.
We are currently evaluating this issue and will update this article when we have more information.