If you are using iOS or Android, please install the most recent version of native app to avoid this problem. See How and why should I install the native App (iOS/Android) instead of using the App in a web browser?
If you are using the web app, for several reasons, it is strongly recommended to keep the app open during the test and stay attentive to the timer. If you leave the app while the timer is running, there is a small chance that your browser or OS will reset the app. If this does occur, your result is still valid as long as you read it 15 minutes after you placed your sample. You won't be able to store the result, but a photograph of your test device provides a good record in this situation.