Thanks so much for writing and letting me know Marko. Mindful tries to offer free cross-device syncing and the syncing-part is one of the main reasons why it’s still in beta.
You can still retrieve your notes with
localStorage.getItem('backup') then you should be able to retrieve your lost notes.
The next update should fix this. Make sure you’re on version 0.9.8204. This will allow you to recover your lost notes. Very sorry for the inconvenience.
Which version are you on currently? Feel free to drop me an email on firstname.lastname@example.org