It could just be the ribbon on the Minute Man's ponytail has been polished away. I did an overlay of the coin to check if it might be a clash mark, but it doesn't match.
Here are a couple of images from PCGS
. It looks like it was a problem area. Lots of images show the details in this area are missing. Click on the photo for a lager image
"Shine, shine, a Roosevelt dime
All the way to Baltimore and running out of time" - Tom Waits 'Clap Hands'