In medium scalability, Lumen Global Illumination is off. So, what you see is distance field Ambient occlusion.
Here are a few general suggestions to take into consideration that I noticed other people had issues with too.
In medium scalability, Lumen Global Illumination is off, so you are left with Distance Field Ambient Occlusion. when you are creating geometry from scratch using the modeling tools, especially with Boolean operations try to not to make them too large in size. the simpler the meshes the better. IF you have a mesh that is too large or is not a closed mesh (air tight)with all its welded vertices and edges you will find some of these undesired effects. Try to split the mesh into smaller chunks and see if that helps. Inspect the mesh with the modeling tools and see if there are any un welded edges.