I was able to get this to happen with a blank setup using just the landscape settings you’ve got. I enclosed my landscape in a lightmass importance volume and saw the components being shaded similarly to your screenshots.
Although, I’m not exactly sure what I did to get rid of it, but I’ve gotten rid of it from my setup. I deleted the lightmass importance volume with my testing and built lighting. At this point I noticed the problem gone. I’ve not been able to get it back, even using my original setup and settings.
Can you maybe try to delete the LMI and build lighting and add it back and see if you’re still seeing the issue?
I’ll continue to look into when I can and see if I can get a more sustained repro. If you come across anything please let me know!