After a bunch of help from Alex it was discovered that a tool called SageThumbs can cause issues with thumbnails showing up in GTV.
SageThumbs is a Windows shell extension that allows file types like TGA and PSD to be previewed as thumbnails in Windows Explorer.
It turns out SageThumbs can cause other issues with Windows 10, some of which are covered in this Reddit post: https://www.reddit.com/r/Windows10/comments/3wl7o9/do_not_install_sagethumb_on_windows_10_it/
Thank you so much Steve for this bug find!