System Information
Operating system: Windows-10-10.0.18362-SP0 64 Bits
Graphics card: GeForce GTX 1080/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 441.66
Blender Version
Broken: version: 2.82 (sub 7), branch: master, commit date: 2020-02-12 16:20, hash: rB77d23b0bd76f
Worked: (optional)
Short description of error
When rendering an emissive transparency on top of a transparent background (either Eevee or Cycles) the transparent object's material color becomes completely desaturated. This only happens in the render (F12) not in the viewport camera.
Exact steps for others to reproduce the error
In Render Properties, go to Film and turn on Transparency.
Create a sphere, add a mix shader with transparent and emissive and make it mostly transparent to see the results better (Factor of .05 or so).
Crank the emissive node up to maybe 50 and give it a color 1,0,0 for pure red.
In viewport camera, you see what you would expect, a red-ish emissive sphere with some transparency. When rendered however, the sphere will be almost completely desaturated of color.
Turn on bloom in Eevee to get the effect even more.
In the viewport, you will see the bloom and it look as it should but when rendered, no bloom and no color.
In this image, the left version is rendered with a transparent background and composited in, the right is rendered with the background in place (no alpha).
https://imgur.com/a/3swdIuf
