It seems the faces of your outline mesh are not pointing inwards. In the attached fbx they do. Make sure Cull Back Faces is enabled and Cast shadows is disabled for the outline mesh. Honestly it's a bit puzzling to me that you couldn't figure this out with all resources easy to find (and linked to). Good luck!
Your front (and top ) doesn't seem to be co-planer anymore.. (scale to 0 in Y and Z direction) or maybe also check the normals (recalc or enable face orientation overlay) or double geometry (merge by distance).. also try make it "cleaner" (?):