In the past, when user fill area under curve in Stacked lines by Y offset graph, they are filled to the same base. See left graph.
In Origin 2020b, on Stack tab of Layer level in Plot Details dialog, a Custom Additional Line by Relative Position checkbox is added. Check it, each curve will be filled to relative additional line. Note: The additional line position is set on Grids tab of Axis dialog. E.g. y=0.