Rreproduces in 2019.4.7f1
Reproduces in 2020.1.1f1
Repro steps:
1. Create a terrain
2. Create a sphere
3. Drag the sphere into the asset browser to create a prefab
4. Create a mesh detail on your terrain using the sphere and paint it on the terrain
5. Click "edit details" and add a grass texture, choose default-particle
expected result: the detail is a solid color
actual result: the detail has a dark spot where the UV seam is