[Shaders] ~ Pixel Depth Offset causing artefacts (no POM)

I’ve made a wavy sand material and tried to use its heightmap to affect the pixel depth offset. The result looks good, but I’m getting black artefacts.

As you can see, by applying the same method to the rocky material I get the same artifacts on the sphere. I’ve tried googling around for solutions, but all the results I get involve POM, which I’m not using, so I have no idea how to solve this issue.

Here’s a screenshot of the shader (very basic).