Does TestFlight delete app data after you close the app on your phone?
Solution 1:
You haven't actually specified what kind of storage, you're using for your data - which makes the question very ambigious.
However, the answer in any case to your question is: no. TestFlight does not delete the stored data when the user closes the app.
You must probably have chosen an incorrect manner of storing the user data. The recommended way to store data necessary for user login is to store it in the Keychain on the phone. This is what your app should do.