I honestly don't know
why it works (or why only certain image-types experience the issue on Windows), but I'll take the win.
Pillow documentation clearly indicates that both file-paths and file-type objects are suitable for opening. There's no indication that one is any "better" than the other (nor have I seen any suggestions online anywhere that the file-type object will work better on Windows). *shrug*
Perhaps the file-type object method leaves the closure to python rather than PIL?