Why does Screen Time show usage when iPhone is off?

If you want to exclude Screen Time restrictions on other devices using the same Apple ID, you can follow these steps:

1. Open the Settings app on the device you want to exclude from Screen Time restrictions.
2. Tap on "Screen Time" and toggle off the switch next to "Share Across Devices."

By turning off "Share Across Devices" for Screen Time you can effectively exclude the device from Screen Time restrictions on other devices that use the same Apple ID. Keep in mind that this will only affect the device you're making changes on and won't impact other devices or users in your family sharing group.

If the "Share Across Devices" option in Screen Time settings is greyed out on your device, then...

Turn off Restrictions. If Restrictions are enabled on your device, this may prevent you from using the "Share Across Devices" option. Go to Settings > Screen Time > Content & Privacy Restrictions and turn off Restrictions, then try enabling "Share Across Devices" again.

Please see the pic below which is self-explanatory

This is completely unusable. As a parent, I have really difficult times controlling my kids usage. And no solution for such a long time. Very disappointing. As you can see from the image, it is counting full time "phantom" usage when the device is off (grey bars) and correct (though very imprecise) usage when it's on. So it claimed that the total usage was more than 16h!!! What a mess...