Trick to bypass screen time limit

Screen time limits which have been set on my childrens iPhones have been repeatedly „disappearing“ (I set them on the parent iPhone and without apparent timing or action they are gone after some time).

I challenged my son (12). Hey (finally) told me his work around: When the time is up he requests „one more minute“. After this he closes the app (swipes it to close). When he then restarts the app the limit is gone. He says that timing is critical. And if it does not work with one app he takes another app that is in the same category (eg games).

He showed me and it actually works. Not sure how to control his screen time now.