System Information
Operating system: Linux-5.4.0-70-generic-x86_64-with-glibc2.31 64 Bits
Graphics card: GeForce GTX 1060 6GB/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 460.39

Blender Version
Broken: version: 2.93.0 Alpha, branch: master, commit date: 2021-03-31 04:29, hash: rBe7f890aa59ad
Worked: (newest version of Blender that worked as expected)

Short description of error
A scene with particles (grass) and a monkey head; motion blur enabled; the render has clear artifacts, similar to fireflies. When animation is rendered the fireflies are quiet distracting. The same scene rendered in 2.83 doesn't show the same artifacts.

Exact steps for others to reproduce the error
Open the blend file attached and render frame 19. You'll see the artifacts clearly. When rendered in 2.83, it's not there.

Blender 2.83 render

Blender 2.93 render

Blend file