Repro steps:
1. Download the Terrain Sample Asset Pack from the asset store and import it
2. use URP
3. Create a terrain
4. change to the paint texture tool and add "Sand" and "Moss" to the terrain (these two layers have alpha channels)
5. Paint sand and moss onto the terrain using a soft brush
6. On the layer settings for both sand and moss enable "Opacity as density"
7. Zoom out from the terrain
Expected result: the basemap blends correctly
Actual result: there is a black segment in between the moss and sand when zoomed out