How to reproduce:
1. Open the "Test" Scene in the attached "case_1199812.zip" Unity project
2. Open the Game window and select the GameObject "Particle System" in the Hierarchy window
3. Observe the dark blending artefacts Soft Particles create when intersecting with the "Plane" GameObject

Expected result: the Soft Particles blend seamlessly with the background
Actual result: the Soft Particles produce a dark blending artefact when colliding with other geometry (see the attached screenshot)

Reproducible with: 2019.2.14f1 (LWRP 6.9.2), 2019.3.0f1 (URP 7.1.5), 2020.1.0a14 (URP 7.1.5)
Could not test with: 2017.4.35f1, 2018.4.13f1 (LWRP 4.10.0-preview)

Notes:
1. I could not test the issue on the 2017.4 stream since LWRP is not available
2. LWRP version 4.10.0-preview does not seem to have a Particle shader which supports the Blending Mode needed for Soft Particles, unable to test on 2018.4
3. Reproducible with both Gamma and Linear Color Space with Unlit, Lit and Simple Lit LRWP/URP Particle Shaders