I have noticed that the Flatten tool does not exactly respect the selected flatten mode.
The first mode (remove terrain higher than the starting plane) will raise terrain that is slightly below the starting plane. (It may rarely also disregard the plane altogether and start eroding terrain lower than the flatten plane as if I was using the erode tool)
The second mode (raise terrain lower than the starting plane to be level with the starting plane) will erode terrain slightly higher than the starting plane.
Expected behavior
The first flatten mode (erode higher terrain to be level with starting plane) should not affect any terrain that is any amount lower than the starting plane.
The second flatten mode (raise lower terrain to be level with the starting plane) should not affect any terrain that is any amount higher than the starting plane.
Try using the block/cylinder brush instead. The falloff of the spherical brush probably causes the flattening algorithm to struggle because of the 4x4x4 voxel grid already limiting things.
I caught an instance of it seriously disregarding the selected flatten mode here.
It shouldn’t be doing anything to the terrain that’s higher than where I started, it should only be filling in terrain that’s lower than where I started.
Here’s more video of the flatten tool being completely botched. Trying to flatten down a bit of somewhat elevated terrain does nothing. While the raise mode raises terrain higher than the starting point, which definitely shouldn’t be happening.
It seems like it’s just completely lost the ability to have sub-voxel precision or whatever that the old terrain tools could do. These tools have been nothing but a downgrade.