5.0.3 Master Pose & Copy Pose distorted Skeletal Mesh


I have a problem with skeletal meshes attached to a character getting distorted. In order to isolate the problem I added a character and an attachment with the absolute bare minimum: Make a cube in Blender, weight paint it to a single bone and then use the exact same skeleton for master pose & copy pose. Exported with Send2Unreal.

:white_check_mark: Previewing animations for the cube backpack in the editor works, character and attachment are the exact same skeleton structure after all.
:white_check_mark: Attaching a SkeletalMesh to my character as a child with a duplicated AnimBP works, but that’s not very efficient for performance.
:x: As soon as I add Copy Pose from Mesh in a new AnimBP using the parent’s pose (same skeleton), the mesh is deformed. In the simplified cube test case, only position and rotation seem to be off.
:x: Same problem happens if I switch the Copy Pose approach to a Master Pose Component in the construction script, we get the exact same position/rotation deltas.

Any idea what could be the problem here?