Worlds uses verts to add lighting information. So any overlapping verts will render as if they are in shadow. So where your pieces meet try to offset the verts where you can or merge by distance. Another thing I've noticed is it does the same shadow effect for verts with multiple edges that terminate at them, I think some call them spokes.
Here's a link to the topology best practices documentation that mentions it: https://developers.meta.com/horizon-worlds/learn/documentation/custom-model-import/creating-custom-models-for-horizon-worlds/best-practices
Hope this helps. 👍
