To get the most out of the content you have stored offline, you should disconnect the app from the internet using 'offline mode'. Otherwise, the application will continue to try to load the map over the Internet, which can seriously disrupt the structure of the offline content.
Using 'offline mode' also means that you don't need to completely disconnect your device from the internet. You can activate this by tapping the tick symbol that appears at the top right of the screen when you view a section of the map that has been downloaded.
Note: If you notice issues with the downloaded map such as missing or out of focus tiles, you will need to delete that download and re-download.