Pool issue: some required audio clips are in the trash !

I’ve found a strange behavior in one of my N5 projects.

This project is working fine unless I try to export to a new location using the “Back up Project…” from the File menu… then some of the audio clips were simply dropped from the project :astonished:

Fortunately I’ve had a backup version of the .npr file, I reopen this one and all is fine again… Opening the Pool, I discover that some audio clips required in this project are in the trash; that should explain why they are dropped when the “Back up project…” feature is used.
I’ve had to manually move these audio clips from the Trash folder to the Audio folder under the Pool window in order to be able to run the “Back up project…” feature successfully.

Now, how is it possible that some required audio clips used in a working project are suddenly in the trash ?

Anyone with a similar issue ?