Steps to reproduce:

1. Download and import attached project.
2. Open scene: main.
3. Play.
4. Select GREEN particle system in Hierarchy.
5. Change Start Speed between 0 and 1.

Expected result: Sub-emitted green particles are always visible on screen.
Actual result: Sub-emitter does not emit particles if their speed is 0 or they are not visible.

Notes: Bug only reproduces if sub-emitter render mode is Stretched Billboard!!

Reproducible on: 5.4.0b2, 5.3.1p3.