This was a known issue in app version 3.0.1 on iOS. Please install the latest app update to get rid of this issue.
Your purchase was not lost and there is no need to repeat your purchase. If you restart the app, your purchase should be recognized by the app if you press 'Restore previous purchase' in the 'Upgrade to Premium' screen.
Our apologies for this inconvenience. Feel free to contact our Support Team if you need more information.