Repro steps:
1. Run scene in attached ParticleBatching.zip
2. After a few seconds click Enable in the Frame Debugger
3. In the Transparent Geometry section highlight the two "Draw Dynamic" calls. The reason for the batch breaking is listed as:
"Objects have different batching keys. This is usually caused by using different vertex streams on Particle Systems, or by mixing Lines and Trails, or by mixing lit and unlit geometry."

Note:
not a regression, happens 5.6 and in 2017

Issue: Batching key not set until a particle emits its first particle. (Richard K)