During lookdev, I use a post process material to bring reference images into the engine to compare against my work. The goal is for the reference image to not be touched by any of the Unreal color pipeline and to look exactly in the viewport (and renders) as it would when viewed straight from the file on disk in an external viewing tool. I've found that when using sRGB reference images I had to uncheck the sRGB option in the texture in order for the resulting image to look correct in the viewport. This seems confusing because the image is definitely sRGB encoded. Using this setup, reference images also look correct in Sequencer renders, HOWEVER, they appear too bright in the new Movie Render Queue renders.
To try to get to the bottom of this I created a test case with a simple post process material (black, 0.5 grey, and white swatches plus a linear gradient) and looked at the output values in the different contexts (viewport, Sequencer render, Movie Render Queue render).
The post process material uses the default "Blendable Location" of "After Tonemapping" to try to be the least effected by unreal on it's way to being displayed.
Here is the PPM, note that the large grey swatch is set at 0.5
Here is the viewport output where I use the pixel inspector to measure the 0.5 grey swatch and get a Final Rendered Color of 0.21..., however, visually, the swatch looks correct.
Here is the sequencer output, where the 0.5 grey swatch with a file source color 0.5 and looks correct visually.
Here is the Movie Render Queue output where the 0.5 grey swatch has a source file color of 0.73 and looks too bright
From these results, I'm thinking (probably incorrectly, sorry Rod):
All that said, I see two problems
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-93427 in the post.