Screentime running while phone is turned off

New iPhone 16 with new iOS 18

While the phone is off (fully powered off), the Screentime app counts this time as being "active". It doesn't mark any particular app as being active, the bar graph is just grayed out for the time period. But the hour bars are fully solid gray during the time that the phone is powered off, and it marks the time as "screen time".

I'm not sure how to fix this. I've already confirmed that "Share across devices" is disabled.