Thanks Alewan 😉
but in the meantime I found the solution !
For thoose who are interested, you have to create a mask if not, indeed its does not work…
So with the Ipad version :
-create a layer and create a black rectangle
-go to the first menu (horny sheet) > Canvas> Transparent canvas so you can get a transparent background.
-create a mask
-With the gradient tool create a radial line tool or another one.
-save in PNG format.
Thanks to your reply I got it !
I did not save the picture at each time ("save a copy") in the menu with the horny page icon on the top left.
I did it and so it works.
So that means that one have not to forget to do it each time.
There is no automatic backup with Affinity, right ?