Seems that the Auto Survey code does some strange things with respect to the heading hold option. If you rotate your grid, the heading hold option retains the original calculated heading. Also, if you enable the cross grid option, it again, uses the same originally calculated heading for all portions of the mission.
I really like the heading hold option when flying survey grids as the aircraft seems to have better flight dynamics and waypoint execution since it doesn’t have to turn around at each end of the grid.