Light Source Angle Input disabled when switching to static light on directional lights

If the directional light is switched from stationary to static, the Light Source Angle Input option is disabled in the details pane.

1. Open Editor (any project, default example map)
2. Select Directional light in Scene Outliner
3. In Details pane, locate Light Source Angle in the Distance Field Shadows submenu.

Light source angle input field stays disabled when switching a light to static