Its probably the way your debugger works. Try putting _dimensions and _dimensions as your watch points if your debugger allows that.
As I can see from your code you are using visual studio.
Brook+ allocates these _dimensions arrays dynamically and visual studio has no way to know how many items has been allocated in these arrays and it just shows the 0th element of these arrays.
stream_A->_stream->_dimensions gives indeed 576 !
Sorry about that, at 5:00 am my time my mind was not at its top!