This week our email server provider, Sendgrid, intermittently experienced a problem where they successfully sent some emails but didn't properly provide us with a delivered event confirming this. For full details see Sendgrid's status here: http://status.sendgrid.com/incidents/xgvxy6dp44vp
We rely on the successful receipt of these events from Sendgrid in order to populate our Activity Feed, email reports, and the "on send" path in journeys.
Yesterday when Sendgrid notified us of this situation we worked with their team to successfully find any affected emails and ensure delivery events were correctly recorded. This means reports and the Activity Feed are now up to date. It also means contacts sent emails correctly moved down the "on send" path in journeys.