We are conducting a study using GPS tracking and we have had 3 participants with iPhones who have had no issues. We now have a participant with an iPhone who is continuously receiving notifications that the app has paused. Why is this happening for just this participant? Is there a way to fix this so it won’t inhibit our data collection? They allowed for all notifications and granted full GPS access.
Welcome to the forum and thanks for your question 
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What you are asking is discussed here. It happens basically because the phone does not have enough resources to keep the Avicenna app running in the background and collect sensor data, that’s why as soon as the participant closes the app, iOS tries to free up some resources for other apps, and therefore terminates the Avicenna app, and the participant receives this notification.
Why it does not happen to other three participants? My strong guess is either they are using a better, more powerful iPhone, or they have fewer apps running.
Please note this has nothing to do with notifications. Notifications as you said are properly set and that’s why the participant gets notified about this event.
Hope this helps,
Mohammad