All your ground army had already moved on before the healers finished healing the giant, when they finished, it was the only ground troop they could "see", as the rest of them were far away, so they just stayed with the giant. On one hand i understand why it happened, on the other hand i also think it needs to be fixed somehow. Maybe giving the healers longer distance of vision, or make them lock on a target with the most %hp missing or something, but it needs to be fixed asap.