"Dotted Lines" Light Artifacts

Hi,

I’m trying Unreal to make a presentation about a house design. It is 7 m x 20 m. I guess it’s relatively small, so I gave it a try.
I know this type of scene is a bit hard because the interior is lit by the sunlight just from above, so I put some spotlights as “portals”. As you can see there are some artifacts below the stairs or corners of the wall. The wall’s lightmap resolution is already 4096. Big enough I think.

So is it because I make the wall too big? Should I break it down to several pieces? I have tried separate some walls, but it just leads to some big light leaks from corners (where different wall meet).

Also tried to find similar artifact at troubleshooting pages, I can’t fine one similar to this.

images:
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corners:
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05
Thank you

Never mind. I solved this issue by making the Indirect Lighting Intensity to 1.0 on all Lights.