PDF Image Icon in Windows Explorer Has Blue Downward Arrow

Reently, my Windows10 file Explorer displaying the PDF preview image thumbnail (View - Large image) started adding a chubby downward pointing blue arrow in the lower right hand corner of the thumbnail. The attached image shows the problem. Has anyone seen this, and have you been able to determine what is causing it? All other documents (docx, jpg, dng, etc.) all display the thumbnails correctly without any "modifications".