This has been an issue for me for more than three years on a PC dedicated and connected to my router. The PC runs nothing but licensed Mach3 v2.63(I think). I seem to get edits in the body of the G-code, but if I add a line forget it. You can open the files, close the files, rename the files, shut down Mach3, run CCleaner to get rid of all temp files, reboot the computer and sacrifice a goat but that line you added will never appear. I always figured it was an amusing little Mach3 quirk.