How it it possible to have engagement_time_msec of 1 millisecond?

Question

This is regarding using Firebase with an iOS/Android app. I have lot of users who have a strange sequence of events. first_open and session_start at exactly the same timestamp. Then user_engagement 1 millisecond later. And that event has the engagement_time_msec property with a value of 1. Then no more events for that user ever again. This is iOS. On Android it is similar except the engagement_time_msec value of 1 is on the first_open event instead of the user_engagement event. At first I was thinking this indicates the app opened for the first time and crashed. But my understanding is that user_engagement is only triggered by actually leaving the app intentionally; a crash wouldn't be able to log the event.

Any idea how this sequence of events is possible?

• first_open

• session_start

• user_engagement with enagement_time_msec = 1

• no more events

I also have plenty of users with a more normal sequence of events. Like first_open, session_start, screen_views, custom events, etc.

So it's not a general Firebase implementation problem.