My coffee is still about to kick in, so please bear with me.
The problem is with the tiny 2d preview, not the "shaderball" 3d preview of the material, right?
A pic says more than a thousand words. It's just that either, I don't have, see or understand the problem bc I tried the map and it looked as expected.
edit: ok. Did some testing. I created a small file in PS. a square with the top left, pure white, bottom, right pure black and the rest 50% gr @16bit linear.
The CRN Bitmap shader gives the correct output. (small 3d&2d prev.) The embedded color looks the same as choosing linear. (still prev views) The render fucks up. There's a visible aberration of the desired glossiness.
To me, it looks like a bug, but I'd wait for others to try it before creating a bug report.