System Information
Operating system: macOS 11.1 / Windows 10
Graphics card: AMD Radeon Pro 5500M 4 GB / GTX 1050ti

Blender Version
Broken: Blender 2.92.0 Beta, February 01, 00:13:03- a9b53daf23ba
Worked: NA

Short description of error

Eevee's AOV passes do not seem to deliver accurate colour or alpha channels when compared to (what should be) equal output from the combined channel. Alpha channels seem to specifically be limited to 0.9844 where they should be 1.

Material setup

Viewing the render of the combined pass (note alpha channel value of 1 at the bottom of the viewer)

Viewing the render of the cube pass (note alpha channel value of 0.9844 at the bottom of the viewer)

This has also been noted by this StackExchange user using a ColorRamp instead of an emission shader, this can be replicated with EXRs output from Blender and read into Nuke.

Exact steps for others to reproduce the error

• Create a new emissive material in Eevee and apply it to an object
• Use a Shader to RGB node to convert to colour data and attach to the new AOV Output node for Eevee
• Configure render settings to output as EXR with the "Standard" (linear sRGB) viewer transform
• Configure render passes to render out the newly created AOV in the View Layer Properties panel
• Render!
• Check the values

Blend file is attached!