Steps to reproduce:

1. Open attached project "Particles.zip" using 5.3
2. Open scene "repro"
3. Select particles to simulate them
4. Try out different view angles and notice that shadows are rendered using current mesh of particles and, as a result, shadows look unnatural (Particle5.3.gif shows the issue)

Correct result:

1. Open attached project "Particles.zip" using 5.2
2. Open scene "repro"
3. Select particles to simulate them
4. Try out different view angles and notice correct behaviour of shadows (Particles5.2.gif)