This issue has been closed as 'Won't Fix' due to an extended period of time without updates. If this issue is important to you please let us know by posting on the AnswerHub or UDN, and Epic will re-open the ticket for further review.
If the width of the details panel is reduced, the lock icon next to the name of the selected actor will disappear off screen.
1. OpenUE4Editor (any project)
2. Select any actor in the scene
3. Move the mouse between the main viewport and the details panel until the mouse changes to the adjustment icon
4. Shrink the width of the details panel
Result:
As the panel size is reduced the Lock icon becomes hidden.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-21456 in the post.