Necrotic bone is a drop from the wither skelleton added by Tinker's Construct.
When the grinder grinds a wither skelleton is only drops regular bones
Just wondered if it was possible for the grinder to do the Tinker's special drops (there may be others for other mobs)

MineFactoryReloaded-[1.7.10]2.8.0RC8-86.jar
CoFHCore-[1.7.10]3.0.1-254.jar
ThermalFoundation-[1.7.10]1.0.0-81.jar

This is something that must be addressed on Tinker's Construct side.