Hi SDeslandes,
We understand you're experiencing an issue with your teen being able to access an app during Downtime and past the App Limit. Are you using a Screen Time passcode on their device? If so, have you enabled "Block at Downtime" and "Block at End of Limit"? These settings are located in Downtime > Block at Downtime, and App Limits > [Limit Name] > Block at End of Limit.
Additionally, we'd also suggest ensuring the app is not included in the Always Allowed section of Screen Time Settings.
Lastly, we would recommend changing the Screen Time passcode as shown here:
Use Screen Time on your iPhone, iPad, or iPod touch
After trying all these steps, if the issue remains, we recommend that you reach out to Apple Support directly. Please reach out here:
Get Support
Hope this helps!
Have a great day.