Looking at the dashboard for app events - we have a serious and large decrease in events starting around July 28, 2016 (for android) as evidenced by this image (below) and this check-in. This appears to have been an update done to the schema but we didn't know about it.
For iOS, it appears that a drop in those events occurred earlier (around March 9, 2016) and have steadily decreased since then.
We'll need to figure these things out and get them fixed.