Audible playing on lock screen not counted screen time

I recently set up an iPad for my son, and implemented a one hour per day audible screen time limit. It appears, however, that when audible is playing an audiobook with the iPad locked (the widget showing up on the lock screen), that this time is not counted, meaning that he can listen to literally 24 hours a day, without counting towards his screen time.

I know this is audio, and not strictly speaking screen time, however, it feels like this is not how the limits are meant to work.

Any advice?