Operating system: Linux-4.15.0-55-generic-x86_64-with-debian-buster-sid 64 Bits
Graphics card: GeForce GT 710/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 390.116
Broken: version: 2.80 (sub 75), branch: master, commit date: 2019-07-29 14:47, hash: rBf6cb5f54494e
Short description of error
Rendering a given scene using a single irradiance volume lightprobe versus multiple irradiance volume lightprobes (both sampling from the same locations) gives different results. This assumes that no other lightprobes are present (save for a single reflection cubemap), and that the influence area of all lightprobes involved covers the entire area of interest. In the case where there are multiple lightprobes, every single lightprobe covers the entire area of interest in its influence area.
The results of this discrepancy can be seen in the image below.
Exact steps for others to reproduce the error
Open up each of the attached .blend files found below. Press [Num0] to move viewport to the placed camera, then press [F12] to render. The resulting render in single_lightprobe_scene.blend should match the right image, and the render in multi_lightprobe_scene.blend should match the left image.
The lightprobe placement and configuration can be obtained from the two blend files. multi_lightprobe_scene.blend contains 9 lightprobes that, altogether, sample from the same positions as the single lightprobe in single_lightprobe_scene.blend, once each. All lightprobes in both scenes have an influence area encompassing the entire room; in multi_lightprobe_scene.blend every single lightprobe's influence area covers the entire room.
The below file contains a simple scene with 1 light probe, sampling from 9 points.
The below file contains the same scene with 9 lightprobes, each sampling from one point (which all correspond to one of the 9 points sampled from above).
The render result with the single lightprobe case is as follows using Blender 2.82:
The render result with the multiple lightprobe case is as follows using Blender 2.82:
Note the brighter lighting pattern in the single lightprobe test case. I don't know if the two cases are supposed to be identical, though.
The system info of the version of Blender used to test this is in the below file.