Your BCO shouldn’t really be questioning a construction detail like this independently unless they are suitably qualified. I would ask your BCO to provide a counter argument to your professional’s assessment as to why the detail doesn’t ventilate appropriately. What, in reality, they would do, is use a checking engineer to validate the detail (I wouldn’t expect a BCO to be qualified to counter - but they could be and if so should provide a reason why your SE’s assessment is incorrect).