This has been around since public launch. The rotation widgets emissive material will cause trouble with the post processing and reflections in the scene. Especially troublesome if you with bloom and eye adaptation. It's a minor annoyance of a number of users. This was originally reported in TTP: 338322, but doesn't appear to have been brought over.
Work around: using the viewport Show flags and disabling bloom and eye adaption work for editing in the viewport purposes.
1. Open UE4 Content Examples
2. Use the ExampleProjectWelcome map that opens with the project.
3. Change to the rotation gizmo
Results: The rotation gizmo will appear in the reflections and under right circumstances can glow.
Expected: The rotation gizmo should appear like the move and scale gizmo.
Head over to the existing AnswerHub thread and let us know what's up.