What is it?

The video does not work for me, could you describe your issue instead?

Sorry, I uploaded as private. Now it’s working.

I watched the video, where is the bug?

light baking is doing something there? if you disable it does that fix it?
from Window/Lighting/Settings, disable auto generate

That fix worked, but will I have issue in future by disabling auto-generate?

if you eventually need some of those baked lights, then you’ll need to bake it or it bakes them before build…

but could try play with the bake settings also, maybe its just some resolution/settings problem…
there used to be some viking village baked lighting tutorial somewhere, and/or maybe you can post screenshots of your bake light settings, quality settings, if someone spots anything there.