A simple solution is to drop your model down on the z axis by the thickness of one layer.
Just thought of another possible cause......on the Mesh dropdown select Separate Connected Surfaces. If the number of parts increases from one....then the issue will probably be solved. The cause.....multiple meshes within one stl. S3D doesn't like them!
PS showing sceen prints of single layers is counter-productive. Attaching a Factory file is far, far better. It allows investigation to be done. You can't experiment with a screen shot!
Hi Doug and thank you for your reply! The part I've quoted above was very helpful.
It turns out that my STL was being separated for some reason in my design software. I think it may be a bug in that software, I'll report and see what happens. Meanwhile, your first suggestion above is golden. Dropping to the first good layer is golden advice.