How is my child still using an old Screen Time PIN to extend app time?

I have comprehensive screentime restorations set up for my kids, and have changed the PIN we use to edit my child’s restrictions.

however, my child can ask for them approve extensions to time on apps on their own phone by asking for an extension on his phone then inputting a pin - this pin is different to the screen time PIN I have as a parent!

I’ve changed my PIN several times, turned both our phones off, but my child can still approve time extensions on their own phone with the same PIN they had before which is not my PIN.

I want to stop this as he can - and does! - approve his own game/app time extensions whenever he likes!

Simon

[Re-Titled by Moderator]

Original Title: My child accesses extra screentime with a different pin