hey i know this is pretty old but can still be an issue.
my quick tipp for a fix. you can use the “CopyPaste Landscape” tool. select your hole landscape with Section Tool.
Fill out the Gizmo to complete Landscape and paste it up (about 5.000 units, or what you need). then hole landscape goes up at onces and you can sculpt back down again.
to get the gizmo brush working, you need to select Copy/Paste and Gizmo Brush. (For Gizmo Brush the Circle is selected on standard. so you need to switch that button first, otherwise the gizmo copy paste will not work).