The sculpting tools produce unexpected results on meshes that have certain existing deformers.
Sculpting tools produce unexpected results on bound skin. To edit a bound skin mesh with sculpting tools, first create a Blend Shape deformer and add a tangent space target.
Use Mesh > Cleanup to remove unwanted geometry from your mesh before you use the sculpting tools.
To display vertex colors while sculpting, turn off Show Frozen.
Hard edges reappear once you exit the active sculpting tool.