Sorry, the crucial element is the presence of an opacity directive.

I have searched online for hints about this but nothing specific comes
up. The only thing that can be remotely related are these problems:

http://tex.stackexchange.com/questions/21011/transparency-in-tikz-preview-package-and-xelatex

https://sourceforge.net/p/pgf/bugs/173/

http://tex.stackexchange.com/questions/17080/does-tikz-2-1-no-longer-work-with-the-preview-package