Setting a PoseAsset to Additive breaks the poses

I have a character face set up with MorphTargets. I followed the PoseAsset creation for the face exactly how the Livestream showed (4.13 Animation Features | 02 | Feature Highlight | Unreal Engine - YouTube). Everything works as expected. But the pose library kind of breaks once I set it to Additive and chose the BasePose (It also happens with the Reference Pose).
It looks like each MorphTarget is being doubled. I’m aware that the livestream demoed this with a joint based face rig, So i’m wondering if there’s something extra to do with a purely blendshape face set up. Any ideas?