Undoing the deleting of variables in use by Sub Anim Instance does not expose them again

If the user deletes variables on an anim blueprint that is used and exposed by a Sub Anim Instance node, and then undoes the deleting, they will not be exposed again, even though they were prior to the delete action.

1. Open QAGame in editor.
2. Create a new anim blueprint based on UE4_Mannequin_Skeleton and open it.
3. Drag off the Final Animation Pose and add a Sub Anim Instance node.
4. Set the Instance Class to AnimBP_SubInst.
5. On the Sub Anim Instance node, expose Head Scale and Copy Bone Alpha.
6. Compile.
7. Open AnimBP_SubInst.
8. In the My Blueprint panel, delete the Head Scale and Copy Bone Also variables.
9. Undo the deleting of those.
10. Go back to the anim blueprint you created.

RESULT

The variables are not exposed again on the Sub Anim Instance.