Hi Rob.
I cannot speak to the specific issue, but first I strongly recommend that you update off of the 4.21 Preview and to the official 4.21.0 engine release. Along with that, make sure that you have the latest versions of both the Datasmith importer plugin in UE4, and the Datasmith exporter plugin in SketchUp.
Then try performing a fresh export and import to see if the issue still persists.