When you are promoted, a skill is automatically added to your next available skill slot. However, it does not visually update with the warning message "Waiting for re-equip" until you click on "Accept". It should automatically update when you open your Class/Weapon Selection menu, regardless of if you click on "Accept" or not. Some might say well, you didn't really "pick it", so why should the visual warning appear? It's there by default, so you've assumed to have picked it.