Typically , key holing happens when there is not enough twist for the weight of the bullet. If you are shooting a known too light bullet the " moment" will be off the center of gravity . Try a lighter bullet or a heavier bullet and see if that makes a difference. Humidity and temp can enter into it as well. Is the keyhole very pronounced or just slight ? The crown, if damaged just a tiny bit , can " push " the bullets butt just enough to make it unstable by the time it hits the target.