I think I found it:
When I add a couple of Visual Editor widgets with Element Spacing/Sidebar Widgets/Bottom set to 0, a blank space appears between the two widgets, even if Element Spacing/Separating Space is set to 0 too. Everything else unites but these two widgets remain detached from each other.
Then with that scenario, if I set Element Spacing/Sidebar Widgets/Bottom to 1, the two widgets merge among them but then some padding appears at the bottom of each one.
You can try by yourself, you have to put just 2 widgets