Grabpass in linear space

I'm working on rendering the thickness of an object, my settings use linear space and HDR, to do this I have a pass that does:

Cull Front ZTest GEqual

and outputs the distance to the camera.

This is followed by grabpass, and then in my final pass I for now render the grabbed pass. What occured to me was that everything looked brighter, and this was solved by applying pow(grabbed_distance, 2.2).

Say I wanted to do refraction, I'd have to do this for the refracted color to be correct (tested by removing the first pass, everything looked brighter still).

So grabpass works in linear space, which is expected when I'm working in linear space, but I also expect that returning the grabbed pass means it matches the outcolor. But instead my shader stays in linear space while the rest of the scene does not...

So I've got the workaround, but I don't understand why I have to do this and feel that it is wrong. If someone can explain more on how this would or should work that could clear things up for me.

Thanks!