I can across this issue when I was Ad Hocing and wanted to pose a skeletal mesh in a certain way. I imported one into my project and moved a bone, then undo and it was gone. I wanted to check if it was the import that was the issue so I went into QA Game to get another Skeletal Mesh.
Note SK_Mannequin does NOT have this issue.
This is a Regression from 4.13.2
Result: The model disappears from the skeletal mesh editor
Expected: The model reverts to its previous position
Note A workaround would be to close and reopen the Skeletal Mesh Editor
There's no existing public thread on this issue, so head over toAnswerHub just mention UE-38430 in the post.