Steps to reproduce:

1. Download and open the attached project
2. Open Scene "TimelineSubEmitter_Bug"
3. Select "Timeline Normal" to open it in the Timeline window
4. Preview the timeline

Expected: all particles, including sub emitters, are spawned
Actual result: only the parent particle emitter is activated

Reproduced on 2017.3.0a1, 2017.3.0b6, 2017.4.10f1, 2017.4.16f1, 2018.2.0b1, 2018.2.18f1, 2018.1.0a1, 2018.3.0b12, 2019.1.0a11
Not reproduced on 2017.3.0b7, 2017.3.0f1, 2017.3.1p4, 2017.4.0f1, 2017.4.9f1*, 2018.2.0a1, 2018.2.0a8
Regressions introduced in 2017.4.10f1, 2018.2.0b1

Notes:
- creating separate tracks for sub emitters with the same seed as parent results in particles being only run for the parent's set length
-*seems like it was fixed for a time and then got broken as some part of a backport (2017.3.0b7-2017.4.9f1 are fine, but versions around it aren't)

Update: fixed in 2017.4.27f1, 2018.3.12f1, 2019.1.0b9, 2019.2.0a9