Settings in Particle System for reproduction:

• Texture Sheet Animation enabled (is not necessary, helps to see the shadow texture)
• Renderer -> Render Mode -> Mesh
• Renderer -> Enable Mesh GPU Instancing enabled

How to reproduce:
1. Open attached project "ParticleTillingBug.zip" and scene "SampleScene"
2. In Hierarchy window, select "SmokeParticleSystem" object
3. In Scene view, observe particle's shadows

Expected result: the shadow of the smoke particle is normaly projected onto the ground as shadow
Actual result: the shadow of the smoke is projected as a sprite sheet texture

Reproducible with: 2018.1.0a3, 2018.1.9f2, 2018.3.0b3, 2018.1.0a2
Not reproducible with: 2017.3.2f1, 2017.4.11f1, 2018.1.0a2