I have to say, I don't think I've ever experienced this. Then again I've rarely manned the guns in marginal weather, and I usually let the AI crew man the cannons. (There's just something godlike about sitting in the conning tower pretending to direct your guncrew
)
I know that's not very helpful, but I just thought it was really interesting that so many others had seen this and I hadn't. If you let the guncrew work the cannon, do they still fire far left/right of the target?