When dumping slag on concrete with a stacker, the grass underneath shows through

probably limitation of engine. all surface materials(dirt, stone, slag, sand, ect.) blend into each other. I'm not sure they can add clear separation.

iirc excavator can mine a bit under designation level( for example mine some dirt and leave bits of slag when you level that pile)

I tested this as soon as I played U1, as I was hoping that this behaviour would change when dumping on tiles.
It also will place some of the dumped material in the dirt layer, and the same volume of dirt gets displaced into the pile.
This is also true of truck-made ramps, not just stacker dumping.