When used in a layered material, it seems that the opacity colour is being multiplied in the diffuse channel when viewing the diffuse AOV.
In the following example, the first image has a simple grey material with a gradient ramp in the opacity slot. When viewing the diffuse source colour, the plane object is a flat grey, which is expected.
Then when I use a layered material with the same grey material as the base, and a simple green material as the first layer, the grey material clearly has the opacity gradient ramp visible in the diffuse source colour, which seems like a bug.
coronaOpacityBug.max (516 KB)