I don't think it's a bug, it's just that reinforcements can take quite a long time getting to the flag. As a rule, I generally move the flag to the map edge, which gives me control as soon as possible, but the cavalry will still be "useful" sooner than infantry. On the attack, unless the battle is one of the really drawn out affairs, I rarely get reinforcements into play. Defence is a little better, I can usually swap out archers and depleted cav without too much trouble.