The default shader currently does not support some effects (like curvature vignetting and rounded corners) for more than one screen or when artworks are activated. But there is a special shader in ./hlsl/artwork_support/ as fallback for those of you who want to use artworks with these effects. Copy both .fx files from there into the ./hlsl/ folder and replace the existing files.
Unfortunately the ./artwork_support/ sub-folder is missing in the binary bundle, so you have to download the source bundle and take the files from there.