Chris, I don't have a BR (though I did have a Storm Petrel with a club boom), so I'm really speaking out of turn. However, looking at your pic I wonder whether the strop from the club boom is twisting the steel stemhead band, thereby eventually loosening the screw, rather than pulling at it? I could imagine that on some points of sailing the twisting force could be considerable.
Something other than a twist shackle, to reduce or eliminate the element of twist, would at least show whether this had something to do with it. Just a thought!
Michael