Oh wait, you’re setting the transform on tick?! That wasn’t in the original screenshot.
It’s attached to the parent (left hand), so you don’t need to set the transform on tick. It will stay attached to a skeletal mesh as long as you give it a parent socket (lf_wrist). Try disconnecting the tick event and maneuvering the Cube Component in the blueprint viewport to the right position (or doing it once on BeginPlay).
EDIT: Basically what I think is happening is the attachment and the tick event are both trying to set the location of the box component, therefore the engine thinks the box is in two locations at the same time.