If I go into the Settings and check "Show FPS Counter", it is displayed while playing and seems to work as normal.
If I quit Insurgency: Sandstorm, and restart it, then take a look my Settings, "Show FPS Counter" is still checked. But when I play, the FPS counter is missing.
If I uncheck and recheck the "Show FPS Counter", the FPS counter reappears and works properly. It continues to work, until I quit Insurgency: Sandstorm.