No book structure after upgrade D7 to D8

I upgrade my website from D7 to D8 (test), with the user interface.
In D7 I have a book with it's structure.
In the D8 website, before upgrade, I checked the checkbox for book module.
And I manage upgrade.

Results :
my book contents are there but there is no structure.
I can manualy create a new book and then link each book contents to that structure, it works, but I think upgrade should do that automatically.
Is it a problem of my upgrade or is it a known fact?