Pretty much what title says. If you have a particle system with prewarm enabled it will start as expected: some particles at different stages of their lifetime.
If you call Stop() and then a while later Start(), the particle system will restart from scratch instead of prewarmed.
This was observed when updating from 2022.3.26 to 2022.3.28.
Furthermore, it only happens if you call Particles.Play() when there’s still some particles active after calling Particles.Stop().
To summarize:
If there’s NO particles alive and call Play after previously calling Stop: The particle system will restart prewarmed.
If there’s any particle alive and call Play after previously calling Stop: The particle system will start spawning at the defined rate but no prewarmed particle will appear.
A workaround is to call Particles.Clear() before Particles.Play(), this will ensure the system will restart prewarmed even if there were some remaining particles.