Hi! Do you lose your snapshots after a reboot, or after you manually close and reopen Mosaico? It could be that Mosaico tries to save snapshots during system shoutdown, but it is somehow killed before being able to complete the saving, resulting in a corrupt snapshots file, hence the lost snapshots.
I’m working to fix this issue. In the meantime you may try to close Mosaico manually and keep a backup of your snapshots for an easy recovery.