To reproduce:

1. Open attached project.
2. Open scene: scene with six meteors
3. Press play
4. Observe how particles crashes into the plane.
5. Repeat steps 3-4 several times, until you get two different outcomes.

Expected result: Meteor always explodes.
Actual result: Sometimes particle subemitter spawns particles, sometimes not. Particles should be emitted by ParticleSystem.Emit(1).

Notes: There is a script ParticleProjectile.cs, which checks if rigid body hits the ground collider, then it tries to do ParticleSystem.Emit(1). Sometimes a flash is visible (direct ps which is played), but its subemitters are not played.

If particles are created at the same time, the same output it will produce. but if they're created at different times, result is random.
Some times half of the subemitters spawn particles, but others don't. (Gifs)

Reproduced on: 5.6.0b6, 5.5.0p1