By chance do we know what rod blanks this has happened on? However unlikely, is it at all possible that a particular blank could expand, even during a one time "stress-relief", for whatever reason? I'm having trouble with the idea that an 8" buttwrap can suddenly contract enough after being wrapped, tied, CP'd, then finished, to cause a 1/2" break. I am still holding to the theory that this is being caused by a combination of things, not just one. Otherwise, it would be a lot more common.

After an experience with the CP just sitting on top of the threads and not soaking all the way through I started thinning my CP to be sure that it would soak all the way through to the blank. I am not sure if this is part of the issue of the cracking or not but if CP is too thick it just won't soak all the way to the blank, and if your CP is not getting to the blank and your epoxy can't becase of the CP sitting on top that may cause some problems.