How to reproduce:
1. Open the attached project "1241626.zip"
2. Open the "Test" Scene
3. Enter the Play Mode
4. Select the "FX Camera" GameObject in the Inspector window and uncheck the "Post-processing" checkbox under the "Rendering" settings of the Camera component

Expected result: no hard edges appear where soft particles intersect with an opaque surface
Actual result: hard edges appear where soft particles intersect with an opaque surface

Reproducible with: 2019.4.2f1 (URP version: 7.3.1, 7.4.1) , 2020.1.0b13 (URP version: 8.1.0), 2020.2.0a15 (URP version: 9.0.0-preview.35)
Could not test with: 2018.4.24f1 (URP version: 4.10.0) and earlier as Camera Stacking is not introduced in these versions

Workarounds:
- Enabling "Clear Depth" option under "Rendering" settings of the Camera component fixes the issue
- Switching Main Camera's Anti-aliasing to None or FXAA - fixes the issue
- Switching Anti Aliasing (MSAA) to "Disabled" under URP Asset Quality settings fixes the issue

Note: tested and reproduced with Direct3D11, Vulkan and OpenGL Core APIs