I confess I do not use the "Library Maintenance" function.
I will.
Well, it now seems clear that the cause of the suppression was not Calibre.
Before opening a topic here, I searched if many other cases exist, and I had not found.
I wanted to be sure.
Now I am : As usual, the problem is between the keyboard and the chair ...
I considere this topic can be closed.
Thanks again for your answers.
@ DoctorOhh :
I copy / past the whole library's directory using files manager. I did'nt run library maintenance. I don't use cloud synchronisation (I use cloud for backup only).