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