Why does my Bridge still show the temperature when I ended the cook on App?

Your phone /App is separate from the WiFi Bridge. It is because you can still read the temperature without your phone. Since the Bridge has its signal and can show you temperature readings.
In general, when you end a cook on the app, the Bridge will still show temperature, which means that the Bridge is still reading the Stick via the Bluetooth signal.
If you would like to change the recipe with the same Stick it will still automatically update on the Bridge LCD as long as they are within range.