I have not had this happen with V4, but have seen it with the mil dragon. With the dragon, it happens with a few particular pose sets, not all sets. And it's reproducible. I suspected that the poses include some lock-down on the absolute location of a body part, while the other parts are relative. I was able to adjust the offending parts and have not investigated further. C6 constraints are still unfamiliar territory to me.
I also found that zeroing the pose and reapplying it simply got me the same (wrong) result! That is most strange.