Appears to not be a bug, just poorly named field, imo. EM2013 screen grab kinda leads to it.
Per an old Apple discussion link and support note (since changed to new/different text), Standby is just how long the phone has not been on a charger.
https://discussions.apple.com/thread/2620391
Standby: Amount of time iPhone has been powered on since its last full charge, including the time the phone has been asleep.
Quick glance at numbers this morning point to this. Pulled the phone from the charger when I got out of bed, both at zero. Started a download of a podcast and got dressed for a workout. Standby was a couple minutes longer than Usage number. Workout while listening to podcasts and using GPS app for tracking. Got in shower, checked afterwards, gap grew a little bit more.
Add in that depending on how many apps you have that are doing background updates, weak cell signal causing phone to ping harder, notifications, phone listening for "Hey Siri", and so on, many people's phones may never really be idle. Add in possibly frequently putting device on charger, never really get to see long-term use numbers.
Guess a really good way to test if a bug is to charge the phone to 100% before going to bed and just put it aside. See what the numbers are when you wake, see if the Standby number has a big gap vs. Usage.