The app only updates the tile every 30 minutes, unless you tap it to launch the app, which then updates immediately. So after you plug in you may have to wait 1 to 29 minutes before the tile changes to charging.
The 30 minute rule is currently a limitation of the OS, but the dev has mentined he may try changing the update cycle time to every 10 minutes, but not sure what that will do for battery life. Actually have my phone running an overnight test right now with all background tasks (and radios) turned off. Tomorrow I will turn only Battery Meter on to get a sense of how much juice, if any, it is actually using.