1. Create a Level Sequence
2. Add a Directional Light to your scene
3. Drag the Directional light into Sequencer
4. Press +Track and choose Source Angle
5. Key the value over several frames
6. Scrub Sequencer in the editor and note that it correctly softens/sharpens the shadows
7. Render using MRQ/Legacy Renderer, or PIE session (with the ALevelSequenceActor set to Autoplay/Loop Indefinitely)
8. Note that the source angle no longer animates.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-142376 in the post.