I am working in this file which is setup normally.
But all the autosave documents are going into the trash.

the log is showing this :

2019-09-19 19:00:24.309 : Updating autosave /Users/yanmaresz/Documents/Dorico Projects/Tendances.dorico → /Users/yanmaresz/Library/Application Support/Steinberg/Dorico 3/AutoSave
2019-09-19 19:00:24.367 : notifyPostCommandExecute: File.Save (1019 ms)
2019-09-19 19:00:25.907 : Executing command: Application.Preferences
2019-09-19 19:00:53.801 : notifyPostCommandExecute: Application.Preferences (27895 ms)
2019-09-19 19:00:55.225 : Executing command: File.Save
2019-09-19 19:00:55.750 : Writing Graphic File Supplementary Data to /var/folders/23/_3qwnfqc8xj09j006s6mmcgr0000gp/T/Dorico 3/ocfsessions/session-b233/1d9f-a012-07c3-9e7c/supplementary_data/graphics/graphicManifest.xml
2019-09-19 19:00:55.752 : Updating engine data path
2019-09-19 19:00:55.752 : Saving document session 0 to /var/folders/23/_3qwnfqc8xj09j006s6mmcgr0000gp/T/Dorico 3/ocfsessions/session-b233/1d9f-a012-07c3-9e7c/supplementary_data/vstaudioengine/enginedata
2019-09-19 19:00:56.012 : writing workspace data to /var/folders/23/_3qwnfqc8xj09j006s6mmcgr0000gp/T/Dorico 3/ocfsessions/session-b233/1d9f-a012-07c3-9e7c/supplementary_data/workspace/workspace.xml
2019-09-19 19:00:56.163 : Deleting old backup /Users/yanmaresz/Documents/Dorico Projects/Backup Projects/Tendances/2019-09-19 18-51-06_Tendances.dorico
2019-09-19 19:00:56.173 : Backup file /Users/yanmaresz/Documents/Dorico Projects/Tendances.dorico → /Users/yanmaresz/Documents/Dorico Projects/Backup Projects/Tendances
2019-09-19 19:00:56.174 : Updating autosave /Users/yanmaresz/Documents/Dorico Projects/Tendances.dorico → /Users/yanmaresz/Library/Application Support/Steinberg/Dorico 3/AutoSave
2019-09-19 19:00:56.231 : notifyPostCommandExecute: File.Save (1005 ms)

May be I’m mistaken and they are backups that are flushed…

There is a limit to the number of backup files per project. When that is reached, the oldest backup file is sent to the recycle bin in Windows (and I guess to the equivalent place in MacOS) so you can still recover older backup files if you need to.

Autosave files are automatically deleted when Dorico ends normally. They are only created to protect against a crash, not as “permanent” backup.

Yes, when Dorico rotates the auto-backups, or when it deletes the auto-saves on a clean exit, it moves them to the Recycle Bin (Windows) or Trash (Mac) so that they are recoverable if need be. Similarly with things like any saved options, if you choose to ‘Remove Saved Defaults’: they all go to the trash so you can get them back if you didn’t mean to remove them.