GPU System will kill existing particles if activate is called after it has be deactivated

Deactivating and then reactivating the same GPU system will cause existing particles to die.

1. Create a Niagara system using the Omnidirectional Burst template emitter
2. Change the emitter's Sim Target to GPUCompute Sim
3. Disable the Gravity Force module 
4. Increase the Drag value to 2 in the Drag module
5. Change the lifetime to 5.0 in the Initialize Particle module
6. Compile and Save the system
7. Add the system to a level
8. In the level blueprint bind events to the 1 and 2 keys
9. Have 1 call deactivate on the system and have 2 call activate
10. PIE and press 1 before the particles die then press 2 soon after

RESULT

The existing particles will be killed, and new ones will spawn. On CPU systems the existing particles will remain and live until their lifetime runs out.