If the Export setting Remove Person Info is selected, a keyword with the Person attribute will be exported if it isn't associated with a face rectangle. To reproduce:
1. Create a keyword John.
2. Edit the keyword and check the Person attribute.
3. Assign the keyword to a photo.
4. Export the photo to disk with Include: All Metadata and Remove Person Info selected.
5. Use ExifTool to verify that the keyword John was exported.
Tested on LR CC 2015.12 / OS X 10.12.6 and Windows 10.