Oppo find X9U proximity sensor issue

When I'm on a call and have the phone near my ear the sensor works fine by turning the screen off. But if I open an app, for example WhatsApp or any other app the sensor does not turn the screen off as it should. For it to work again I need to go to the call itself for the sensor to work. Any fix for this issue or is it an option that I need to toggle. Appreciate any help.