A. To prevent the app from losing your place, adjust your iPhone's auto-lock settings. Go to 'Settings' > 'Display & Brightness' > 'Auto-Lock' and set it to 'Never' while you are using the app. Just remember to change it back afterward to save battery life.
B. If you prefer not to change your auto-lock settings, try to keep interacting with the app periodically, such as scrolling or tapping the screen, to keep it active.