Sketchup Scenes Go Dark in Layout Viewport

After updating to Sketchup/Layout 2023, some of my Sketchup scenes (south facing sections) go dark in Layout Viewports as if shadows are on. All shadows are off in SU and LO. This only happens in parallel projection in viewports, not with perspective. Can’t sort out why this would be happening. Seems to be new with SU/LO 2023. What am I missing?

If you upload the .skp file to the forum someone will figure it out. If the file is larger than 16MB, place it in a file x-fer service like Dropbox or Wetransfer and post the link.