How to reproduce:
1. Create a project with LWRP template
2. Create a new Scene
3. Create a cube and a Particle System
4. Enable Depth Texture in the Render Pipeline asset
5. Create a Material using LWRP/Particles/Unlit shader, set the Rendering Mode to Transparent and enable Soft Particles
6. Set the Particle System to use the created material
7. Enter Play mode

Expected results: Soft particles work and there are no hard edges between the cube ant the particles
Actual results: There are hard edges between the cube and the particles

Reproducible in Unity versions: 2018.3.6f1, 2019.1.0b4, 2019.2.0a6