Hello
Here is a screenshot which illustrates the issue.
When I create an empty shape (eg: empty rectangle with 10px thickness) in Adobe Illustrator and import it in FontForge, FF fills it which is not what I want. Seems not possible to change this behavior in FF (or if so tell me please !) but when editing the glyph in Illustrator maybe it is required to use some attribute to avoid FF filling the shape (my spouse suggested it could be the Alpha layer)
Comments
In fact the 2 rectangles in Ai where "filled with white"
Problem solved