For SOME broadcasts, we are currently having an issue to record SENT stats for broadcast. You do not need to worry about email sending and deliverability. The affected broadcasts have been sent to ALL your recipients but the stats sent are not fully recorded. All other stats (open, click, and etc) are working fine. You may already notice this because your open rate will be higher than the previous broadcast.
We have identified the issue and currently working to fix it.
Again, this happens to SOME broadcasts only. Not all broadcasts are affected.