Staggering Palm + Killing Palm Bug

When you have the Buff from Staggering Palm that fires a projectile, you can no longer use Killing Palm to generate Power Charges. Even if the the culling mark is above the mobs head, killing the mob with Killing Palm does not generate a Power Charge while the buff is active.

*EDIT*

This apparently does not happen all the time. Not sure what causes it to not generate the power charge.

*EDIT 2*

Think I got it now. For this to happen
Staggering Palm Buff needs to be active
The mob must have both ready to Cull and Stagger marks

It still does not seem to happen every time, but most of the time, so it is very repeatable.

Use Staggering Palm until you've gotten the buff from one mob
Use Staggering Palm until another mob is ready to be staggered or is staggered, with both culling and staggering marks
Use Killing Palm on that mob

You will charge at them, hit them, kill them, but not gain a charge or activate support gems such as Life Drain. This leads me to suspect that it is the projectile from the buff hitting the mob first as a separate skill, that is killing them before the skill you used to attack hits them, such as Killing Palm in this instance.