Create a skeletalmesh LODSettings asset by using an existing skeletalmesh
Import a skeletal mesh
open the skeletalmesh editor, Add the LODSettings and hit regenerate button under the LOD Settings category
reimport one of the LOD created by the LOD Settings has a custom LOD (you can use the same fbx you use for the base skeletal mesh LOD)
RESULT: The imported LOD is imported but the LODSettings is overriding it with the settings so you do not see the imported LOD
EXPECTED: The LODSettings stay and the custom LOD is overriding the generated LOD
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-93107 in the post.