Enable_HandIK_L or R only updates when playing in BaseLayer slot?

[ShreddinPB]

Hey guys, I use IK to attach my characters left arm to the weapon. I am then trying to play the reload animation in just the Arm_L slot. I have added the Enable_HandIK_L and Layering_arm_L_Add curves to the montage. If I have the slot set to BaseLayer, it turns off the IK and plays the animation, only it plays it on the whole body. If I switch the slot to just play on the arm, the IK values no longer update and the arm stays IK'd to the weapon.
Do I need to add more curves? I have tried many different combinations of curves and slots and can only get those values to update if the slot is BaseLayer.
What am I doing wrong? Thanks!