Welcome to Android Central! Are you certain the tablet has a smartcover sensor in the first place? I'm not sure if the Tab A line does -- my previous Tab A7 lite didn't. It's not unusual for cheap 3rd party case sellers to lie and claim that a case has smartcover ability.
The easy way to check if a device has a smartcover feature is to slowly pass a magnet over the entire bezel of the tablet, since smart cases use embedded magnets in the flip cover to trigger the magnetic sensor in the bezel. If the screen turns off when you pass a magnet over a specific part of a bezel, then that confirms the device has the feature.
I tried searching the web to see if the Tab A9+ has the feature, but couldn't find any specific mention.