i like to point out that some areas of tile can't be touched, i think it a bug. i was using the flatten tool in sculpt mode and this corner can't be flatten.
Hard to tell, is it at the join between two tiles? You can try right clicking one and select Stitch adjacent tiles.
The flatten and smooth tools do not always work on the terrain. But the Sculpt tool does. Either use it as it is to create hills or using the key modifier [Shift] to create holes. Try this. Also is this a problem at the crossover between Lidar and non-Lidar tiles?
Ah, I understand it now. The tools are not working at the tile. Hmm, Interesting. Maybe this specific tile is not properly assigned to a landscape level, so the tools won't work on it.