I have isolated a fragment of the pdf attached here: http://bugs.ghostscript.com/show_bug.cgi?id=691218 MuPDF renders it wrong. Speculation says it's related to Screen, but I wonder...
Created attachment 6968 [details] A simplified test case Adding the test case that is mentioned.
Created attachment 6987 [details] Plain and simple example I think I figured this one out. Softmasks do not apply on paths. At all. And this pdf examplifies this. The file is shown as a blue "outline" text in Preview.app and is completely white in mupdf.