All data is stored on our servers - nothing will be lost.
What happens when your Premium subscription runs out is that the app is set back to the basic functions but all data tracked is still on our servers.
So if you e.g. happen to have a Premium subscription that runs out before you realize it and the app is set back to basic: all data is still there. You can still access all items saved and created under Favorites you can just not create more as the basic app is limited when it comes to creating content under Favorites.
As soon as you are back to Premium you will have the same access/data as before - even if you have been a basic member between Premium subscriptions.