Repro steps:
1. Import the attached package.
2. Open the GPUSortingIssue scene and observe the AddMeSecond system doesn't have many particles
3. Delete the AddMeFirst system (or disable its GPU sorting)
4. Observe now all of the AddMeSecond system's particles are shown.

Notes:
- Regression, does not repro in 2019.2 / package 6.8.0
- If the issue does show right away, remove both system from the scene, and add AddMeFirst followed by AddMeSecond (video attached).
- Note GPU events are also broken