Why only one backup file is shown in Dropbox directory while there are multiple Dropbox backups in Plesk interface?
When Dropbox backend is used for the cloud backups, it keeps overwriting the same file
server.servername.example.com.tar in Dropbox directory, instead of creating new files.
This behavior is by design: all additional backups are stored in the Dropbox revisions. These revisions can be found in Dropbox > backup_location > server.servername.example.com.tar > ⋯ > Version history:
On restoration Plesk Backup Manager will automatically select the correct revision from the list, download and extract it.