Repro steps:
1. Open Scene1 from attached project.
2. Select Particle System and in the scene view observe how it simulates.
3. Press Pause and then Play
4. Exit play mode
5. Select particle system and try to simulate it.
6. Observe how the playback time does not change.

Note:
- Entering / exiting play mode again fixes the issue.
- Not a particle system issue per se, though it's easy to see the issue with it.
- Issue introduced in 5.5 and still happening in 5.6. Not reproducible on 5.4.3f1.