TL:DR - seems there is a conflict when the system initiates snapshots - caused the latest, manual, full snapshot to be purged.
Updating my system and took a full snapshot, then updated an Installed add-on (via supervisor) which triggered a partial snapshot of that component (I forgot to switch it off).
Then tried to take another full snapshot from your WebUI. This indicates it is a partial snapshot not a full snapshot though the logs seem to indicate a full snapshot (still in progress).
07-28 16:39:16 INFO [backup.ha.hasource] Requesting a new snapshot
07-28 16:39:16 DEBUG [backup.ha.harequests] Making Hassio request: http://hassio/snapshots/new/full
07-28 16:39:51 DEBUG [backup.ha.harequests] Making Hassio request: http://hassio/addons/self/info
07-28 16:39:51 DEBUG [backup.ha.harequests] Making Hassio request: http://hassio/info
07-28 16:39:51 DEBUG [backup.ha.harequests] Making Hassio request: http://hassio/core/info
07-28 16:39:51 DEBUG [backup.ha.harequests] Making Hassio request: http://hassio/supervisor/info
Once finished, it said ‘full’
However, the last full snapshot I triggered manually, seems to have been deleted! (this is the most worrying one!)
I did see it in the log file, but the log of that has gone as I have thousands of these messages
07-28 16:48:44 DEBUG [backup.worker.watcher] Backup directory modified event
07-28 16:48:44 DEBUG [backup.worker.watcher] Backup directory changed
07-28 16:48:44 DEBUG [backup.worker.watcher] Backup directory modified event
07-28 16:48:44 DEBUG [backup.worker.watcher] Backup directory changed
07-28 16:48:44 DEBUG [backup.worker.watcher] Backup directory modified event
07-28 16:48:44 DEBUG [backup.worker.watcher] Backup directory changed
07-28 16:48:44 DEBUG [backup.worker.watcher] Backup directory modified event
07-28 16:48:44 DEBUG [backup.worker.watcher] Backup directory changed
07-28 16:48:44 DEBUG [backup.worker.watcher] Backup directory modified event
07-28 16:48:44 DEBUG [backup.worker.watcher] Backup directory changed
07-28 16:48:44 DEBUG [backup.worker.watcher] Backup directory modified event
07-28 16:48:44 DEBUG [backup.worker.watcher] Backup directory changed
Great add-on though 
[edit]
After uploading the new full snapshot, it deleted the partial snapshot!
07-28 16:56:28 DEBUG [backup.drive.drivesource] Uploading Full HASnapshot 2021-07-28 16:39:16 100.00%
07-28 16:56:28 DEBUG [backup.drive.driverequests] Making Google Drive request: https://www.googleapis.com/drive/v3/files/xxxxxxxxxxxxxxxxxxxxxx/?fields=id%2Cname%2CappProperties%2Csize%2Ctrashed%2CmimeType%2CmodifiedTime%2Ccapabilities%2Cparents%2CdriveId&supportsAllDrives=true
07-28 16:56:29 INFO [backup.drive.drivesource] Deleting 'addon_7ad98f9c_zigbee2mqtt_1.15.0' From Google Drive
07-28 16:56:29 DEBUG [backup.drive.driverequests] Making Google Drive request: https://www.googleapis.com/drive/v3/files/xxxxxxxxxxxxxxxxxxxxxxx/?supportsAllDrives=true
07-28 16:56:30 INFO [backup.worker.watcher] Backup directory changed
07-28 16:56:30 DEBUG [backup.model.syncer] Sync requested by Backup Directory Watcher
07-28 16:56:30 INFO [backup.model.coordinator] Syncing Snapshots
07-28 16:56:30 DEBUG [backup.ha.harequests] Making Hassio request: http://hassio/snapshots