I think samsung have auto saving notes even while you close not to save it. if lost it you can recover it bro, and its wierd if your notes just lost not because you delete it. I am sugesting try to cek your device in samsung service center🙏🏼
maybe you can sync the notes under settings-accounts and backup-samsung cloud-other synced data-samsung notes-sync now. Or next time use google keep to make sure your notes are synced to the cloud and accessible from any device
