We’ve had something similar happen, what I figured out is that having ChildActors that are responsible for lights in another blueprint do not seem to like it very much. Additionally another thing that I noticed is that if you having some objects parented in the level then it tends to break lighting as wel and invalidates them.
If you enter “DumpUnbuiltLightInteractions” in the console(output log bottom) then it should give you a list of all things that are currently unbuilt in the level. Maybe it can help you find out exactly why it is happening.
In general I think one of the issues might lie in the fact that when you open a level or play it the construction script is executed and if the light for some reason is just off by 0.0001 in it’s position or rotation it invalidates the lighting. There is an option in C++ where you can adjust that threshold of checking when it should invalidate lighting. Not sure exactly where that is, I’d have to ask our lead programmer. Let me know if you want this info or if any of the previous info helps you out.