A couple things to consider:
1. With the new app, one of the features is to allow the credentials to be remembered. Have the user check their lock timeout setting. Go to the Tools menu (upper right hand corner icon that looks like 3 horizontal lines) and select Settings. Is the Lock Timeout set to "Never"? If so, this means that the app will keep the user logged into the app, even when it is closed. Update this setting to "Always" or to a set time period.
2. To change users, the app needs to be closed - not just minimized but closed. When naturalForms is open and the home button is tapped once, the app is minimized but is still running. Therefore, when the app is tapped again, the user still has a session going but may need to enter their password again (depending on the Lock Timeout setting). Double tap the home button and then swipe the app upward to fully close the app. Then when the user taps on naturalForms, the app will ask for the log in information to start a new session (as long as the Lock Timeout setting is not "Never").