Good Afternoon SaltyGoose,
We see that you're using Screen Time but seeing that an app is still able to be used beyond the time limit, and we understand how this can be counterproductive so you've come to the right place for assistance with this.
Are you aware if a message to "Ignore Limit" is shown on your child's phone when they're using the app?
If you look at the Screen Time usage between the devices, are you seeing that the reported usage is the same and syncing properly?
Also, we'd recommend taking a look at the following resource just to confirm that the settings are set to your preference with using Screen Time:
Use Screen Time on your iPhone, iPad, or iPod touch
You'll want to make sure that you have enabled the "Block at Downtime" / "Block at End of Limit" options for your child's phone and that your Screen Time passcode isn't know by your child. In addition to this, double check to make sure that the app hasn't been included in the Always Allowed section of the Screen Time settings.
We look forward to seeing if these steps help to resolve what you're experiencing.
Have a great day!