Pose Driver affecting child bone rotation

I apologize if this is posted in the wrong place, I’m a total noob in UE.
I’m a rigger in Maya and I want to bring my PSD correctives into UE. I have a pose asset with the base poses I want to track/correct, each pose is the arm extended into a cardinal direction (up, down, forward, back) and a corresponding corrective blendshape target is keyed with the arm. When I hook up that pose asset to a pose driver and track the upper arm bone it seems to over write the animation of the child bones (ie the lower arm).

Any insight would be great!
On the left is the animation with a the pose driver connected, on the right is the animation being fed into the pose driver.