The difference between "testbone" and "testbone2" is only whether there is "joint7"
Skinning itself has been properly ruled out.
However, it can be confirmed by attaching a socket etc., but the bone calculation itself is not excluded.
Add LOD1 to SkeletalMesh
Add "joint2" to "Bones to Remove"
Pressing "Regenerate LOD" will properly remove the bone from "testbone2".
However, some bones remain from "testbone".
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-91023 in the post.