Steps to reproduce:

1. Create a new scene
2. Create a terrain by right clicking in the hierarchy, 3D objects, Terrain
3. Select the new terrain object, and in the inspector window, go to the pain Terrain tab in the terrain component, Select paint texture, and create two new Terrain layers.
4. Use red (1rst layer) and blue (2nd layer) textures for the test. The blue texture has an alpha with various gray intensity.
5. Add the two terrain layers and check "Opacity as Density" for the blue layer.
6. Paint the blue layer with a low Opacity.

Actual results: Opacity looks not really took into account and especially the "Opacity as Density" feature doesn't work. Notice the blue square appearing.

Expected results: When the blue layer is paint with a low opacity brush, more rectangles should appears more the terrain is painted. No blue square appears

Reproducible with versions: 6.3 (URP), 2021.3.55f1, 2021.2.19f1, 2020.3.20f1, 2020.3.8f1

Not reproducible with versions: 2020.3.7f1

Can’t test with versions: 

Tested on (OS): Windows

Notes:

• This Terrain feature works like the "Use Opacity map as Density map" in the HDRP Layer Lit shader. It manage a threshold via the alpha intensity.