It looks like a missed subcase. If the vehicle is in bypass of the same hexside of another obstacle in an adjacent hex, C.5B would require the gun to change CA. I think the same thing should apply even though the vehicle is in bypass of the firer's hex. There would still be the case E penalty in one situation vs. the other, but we can put that down to the vertex being a convenient place to trace LOS and not necessarily the true position of the unit in question. I also think the other hex that has to be in LOS has to include the full hexside that is being bypassed, despite the fact that LOS is traced to just the vertex, i.e. you can't choose between the two hexsides that bound the vertex.
JR