I'm kinda late to this thread, but I think that it is still worth pointing out:

The first attached picture in post #1 clearly shows that the grain burned below the casting tube and there is also what appears to be a bubble on the face of the grain. This would explain the cato. The plastic threads in the Pro38 are designed to let got in this case, so they probably worked as intended and are not to blame. This is also the reason why switching to metal could be problematic. Depending on things like tolerances, it can be tricky to engineer a metal closure that reliably limits pressures to similar levels as the current solution.