Scene capture affected by post process

A SceneCapture2D camera is looking at a plane and renders it to a 256x256 target texture. The scene capture actor has all post process settings turned off, ie all the intensity values at 0, tints and other colors on white, etc. The only thing it does have enabled is a single blendable material which only has a SceneTexture:Specular node. So the scene capture actor is supposed to render exactly what the specular value of the plane in front of it has.

However it is not the case: the captured image is modified in some manner. Take a look at the following image:

It has 4x4 squares. The top left group is the original test pattern with values 0, 0.1, 0.5 and 1.0. The top right group is the captured image. Bottom left is their difference while bottom right is the difference amplified.

The noise made me think that some post process volumes are affecting the capture actor in spite of the Post Process Blend Weight value being 1. I went through all post process volumes in the level and turned off grain yet the noise remained. I confirmed that the capture actor’s post process setting overrides actually work by turning on features: when I set grain intensity to 1 the the noise showed up more clearly. When I set tint to blue, the captured image turned blue. So the overrides work yet the captured image is still fails to show the original values.

I can confirm this issue still exists in Unreal Engine 4.18.1 what’s going on?