If you were using SHM I would have recommended using Intruder Alert with Actions where you can add an entry delay, but since you’re using ADT it should work out of the box with your ADT setup. Maybe try to reset it and see if that helps otherwise you may need to contact ST support to see why the ADT delay isn’t working.
You may also want to confirm that it’s your ADT system triggering the alarm and not some other custom automation rule triggering the alarm.