Reproduction steps:
1. Open the attached project "testParticles.zip";
2. Open the "demo scene";
3. Select "Particle System" object and try to disable and re-enable "Looping" option;
4. Run the scene and then stop it;
5. Select "Particle System" again.

Expected results: After stopping a play mode, particles should still appear in the Game View, or start appearing if you select on the particles object in the Hierarchy.
Actual result: Particles do not appear after closing the play mode, start appearing only if you go to the Scene view and select "Play" in the Particle Effect menu.

Reproduced on: 2018.1.0a3.
Not reproduced on: 2017.3.0b9 or earlier