Why can't the new landfill types be mined?

I tried! I couldn't get this to work I'm sorry. If they can be mined, then they can't be painted over top of generated terrain.

I could possibly make it work by having two versions of the tiles - the auto generated version as well as a placeable/minable version. However, I am cautious of this idea as there is a hard restriction on the number of tile types that can exist.