So when switching to linear color space the camera background gets washed out.
I modified the background shaders so the camera image gets transformed correctly back to Gamma Space and looks correct.
The Shaders are located under ARDK/Rendering/Ressources
ARKitFrame and ARCoreFrame are the ones that need to be replaced with the version below.
The Camera Image now should be looking correct again and you still can use all the Post Processing stuff why you probably even changed to linear in the first place
The shaders can be replaced even if you use gamma, these have shader defines and will switch automatically to gamma or linear behaviour.
Also. I think this switching should be implemented in the ARDK Master build