Hi Ed,
So I actually stumbled onto the source of this. Within the Engine Content under \BufferVisualization\ folder you’ll find the various post process materials used to display the GBuffers. If you open the one for Roughness you’ll see it is going through a power(2.2) node (approximating an sRGB to linear color conversion).
As expected from the experiments described above none of the other gbuffer visualization materials do this. So I really can’t say why this is this way, maybe there was a good reason. But the down side is that these materials are also used for the Render To Texture Toolset.
A work around is simply to bypass that power(2.2) node as shown while using Render To Texture Toolset as shown in this screenshot.
When you do that, the four material attribute HDR images output by the RTTT look correct.