Overview

If HeightFluid and InkCanvas have different NormalMapPaint settings, no normal map is set for material, and HeightFluid generates RenderTexture for NormalMap, it becomes black image and it becomes gloomy shading.

Countermeasure

When doing NormalMapFluid with HeightFluid, if the normal map is specified for the material, you can leave it as it is now.
If you do not specify a normal map and want to do NormalMapFluid, you need to fill in the RenderTexture with the default normal rgb (128, 128, 255) for RenderTexture after generating it. It can filled with SingleColorFill. When treating it as a DXT 5 mn format when calculating Normal Map in Shader, define Pack Normal function in C # (defined in Shader) and apply it to rgb (128, 128, 255) and initialize with that color.