No $duration when using time() method in Swift

Mixpanel.mainInstance().time(event: "Video Played")
player.play()
// some time later...
Mixpanel.mainInstance().track(event: "Video Played", properties: [
        "Video": videoName ])

I’m using the above code to track a duration of an event. I don’t get any errors when running the project and I see that calling 

Mixpanel.mainInstance().eventElapsedTime(event: "Video Played")

… Also returns a valid value for the event duration.

However, when I look at events in the Insights section and try to add a Breakdown by duration, I don’t see any field. Nor do I see $duration value reported by inspecting the Live View. Any ideas what I am missing here