Split body animations using anim slots and additive settings not working properly

I have a question about doing split body anims using layered blend per bone and additive settings in the anim sequence. We have it set up so upper body anims such as fire and melee attack use a specified slot for upper body, and those anims are set to use additive, with the mesh space and first frame of a full body idle animation. It seems to work alright for certain movements like walking, but in some other cases such as sprinting or jumping, it seems like the blending is not working properly for some bones in the upper body. Oftentimes it’s the hands that curve in or out in an undesirable fashion.

For instance, we have the jump + melee – the melee animation uses the left hand, while the right hand is holding the weapon. The melee anim has additive set with the idle animation, and uses upper body slot. I can see that the hand that moves in the melee animation bends in an odd way when you melee in mid-jump.

Any advice will be helpful. Thank you.