Are you using GuixSD or Guix on top of another distro? It seemed to me that when my caches were empty the first application I'd run would cause the shaders to be rebuilt with whatever version of llvm that application was built with. And it also seems that the issue goes both ways, so if you run an application that builds the shaders with llvm6 then all applications which expect 7 will show artifacts, and vice versa, if you clear your cache and then immediately run a program which builds the shaders with llvm7 then any programs which expect llvm6 will see artifacts.
So it sounds to me like you have a combination of programs built with these conflicting versions. In my case, since I'm running GuixSD, I just did a full upgrade, cleared both caches, and then restarted my machine. Haven't had an issue since. However, if you're using just Guix on another distro, then perhaps the host distro software uses llvm6 while your Guix applications use llvm7? If that's the case then I'm not sure what the best solution to that would be. Perhaps you could just consider re-installing the affected Guix applications with: guix package -i your_program --with-input=llvm=llvm@6.0.1 until your host distro updates to llvm7.
