Now that I wrote about this in another thread, I have come to believe this is a bug. On several occasions, the generals of my reinforcements joined the battle despite me already fielding 20 units. One time, the game crashed after this happened (when I tried to direct the incoming 21st unit). On three other occasions, the generals got to the field and even showed up on the unit card panel. The weird thing was that their unit cards covered up some of the cards of my original 20 rendering the original units inaccessible via the unit card panel.