Hello there @Jedm_NZ!
That’s is not an expected behaviour, or even the way the effect should appear in viewport. It sounds like a component in your scene is not loading as intended. I would suggest creating the same light setup in a clean project, to confirm if the issue is tied to your scene, or UE itself. Additionally, please test clearing out the contents of Saved and Intermediate folders in your projects, as this wil force shader recompile, and clear all caches.