Awesome you figured it out! Strange to hear it was the shader but hey good job troubleshooting

I’ve only got basic experience with Blender’s grooming system, but here are my would -be checkpoints:

1. Check the UV map is in good condition, that those bald polygons are properly mapped;

2. Check normals are facing the right way for the entire surface

3. Check the hair isn’t poking through the other side or lying parallel to the surface, which seems obvious and you probably already did;

4. Check you are displaying all hair curves on your viewport, that your limits are enough for the amount of curves

I’m no expert but these are called Fairy Circles iirc. Apparently whatever substrate they grow on is consumed, so they expand in a circular fashion- think of burning a paper starting from a point in its center.