I think this has changed in 1.07a from previous releases. Before, when an army stack was marching to the sound of guns in support of a Corps it wasn´t really involved in the battle, now it takes the brunt of casualties. Here for example, Grant march to the sound of guns to support Meade, and of the Union units eliminated all are from Grant stack, in fact all the units in Grant stack are eliminated.
Is this by design? It seems contrary to game design philosophy, as it would encourage to fiil up army stacks with combat units.