Erosion tool behavior has changed !?

Hi, I’m using the erosion tool to shape my landscape.
Usualy this tools move landscape vertices which are above the angle of the threshold value. Vertices bellow this angle should not be affected by erosion. Meaning that if you put a threshold value of zero, it should act like the “flatten” tool , not moving vertices of an already flat landscape up or down. On a hill, with a higher threshold value, it should create a slope of an angle which depends of this value.
Like in the video from the UE4 doc :

But now, without touching any setting, without any engine update (still using 4.20), the tool acts like the noise tool, moving every vertices, even if the landscape is fully flat and has no elevation to erode.
I tried to recompile the engine but the behavior is the same. Same with a new map.
Can anyone comfirm if using the erosion (not HydroErosion) tool on a flat landscape move the landscape up or down?