Question 3D Generic rig deforms when using additive layers

Hi,

i have this first person model of hands for an FPS shooter

the hand bones are not parented to the arms to make IK work properly

this is what happens when i try to use an additive animation on top of the base layer (anims are just a test to exagerate the problem)

For some reason it didnt capture the layer settings little window :/.
You can see how when i enable the additive layer the hand just goes way out stretching it badly, but if use it as an override layer you can also see how the animation itself is fine.
It kinda makes sense with the hands not being parented to the arms, but i have not idea how to fix it.
I followed a tutorial on how to make a TGT rig, which basically means that in the same rig you duplicate all the deforming bones and make them non-deforming, then constrain all the original deforming bones to those TGT bones so that they follow the TGT bones transforms. I did this hoping to solve the issue, and unless i messed something up in the process it doesnt fix it.

Is this a known issue? I'd love some help, thanks in advance.