The Lumen workflow has several issues. One such an issue is having spots in the scene that, when looked at, make the screen become pure black.

Lumen indoor scene goes completely dark if lights are off-screen - Development / Rendering - Epic Developer Community Forums (unrealengine.com)

And on the road to tackle this issue, one encounters two additional issues.

• It is 100% a lumen issue.
  • In this link you will also find someone suggesting changing exposure to "fix" it.
    • this is obviously wrong. as changing the graphic settings to not use lumen will be completely broken by this.
    • so fixing some "super dark spots" of lumen would be "fixed" by breaking those PP zones for weaker hardware COMPLETELY.
• The actual issue is a) lumens bad bounce light implementation: Real World lux values (sun, etc) vs Not - Development / Rendering - Epic Developer Community Forums (unrealengine.com)
  • lumen has barely any bounce lights wether using 10 Lux or 120000 Lux from a light source.
  • using the lumen debug view in the dark spots, shows that everything is dark in lumens view.
• the second problem b) is lumens bad physical accurateness Lumen GI and Reflections feedback thread - Development / Rendering - Epic Developer Community Forums (unrealengine.com)
  • lumen cannot handle physical correct values (like 70.000 Lux from the sky). it completely overshoots.
  • but the rest of the rendering stack is implemented with physical values in mind
  • the result is that the relative brightness between lightsources can never be fixed when using lumen and there is no exposure setting that solves that.
  • (also one result is that a lumen scene cannot use the same values as a lightbake scene, which is completely stupid from epic)

In a real and finished game, the only viable solution with lumen as it is implemented in 5.0 - 5.4 is: getting rid of it for the final build. relying on traditional lightbaking. This is the only way to a) get a high quality result b) solve all this weird real-time-calculation bugs c) improve perfomance d) improve visuals for lower hardware

If light baking is not a valid option (e.g. technical limitations, highly dynamic environment,….) the only good option to prevent blind spots is a compromise:

• Make the sky as bright as lumen allows it. sure 70.000 is too much for it, but around 700 should still work.
• This should provide a way better intensity span than setting super bright things to intensity 1 or 7 only.
• While it forces to readjust every light (and volumetric fog, and PP volume) in the scene again, in my test it highly reduced the amount of blind spots.
• But be careful too high values make lumen bleed through solid walls.