Just checked out my 10mm Witness Steel and like you had a bear of a time getting the FPS off!
when I got it off I noticed a lip like yours but not as pronounced
Interesting thing is I removed the FPS a while ago to do some polishing, and don't remember it being this hard to take off. Could just be my bad memory, or it could be that the lip forms from wear over time. Either way, I doubt anything bad would happen if you file it away.