Bump since I think the same thing happened to me, see attached.

I have no idea how this happened. It's an old used fork with a basic, bought-new headset. I've commuted daily on the bike for over a year since I replaced the headset. Suddenly it started loosening up while I ride. Tightening the top nut just hits the stop, even after I added two spacers to the stack.

I'm willing to replace the fork, but I think I might have a hard time finding a shop that stocks used parts around here. Would this eventually stop loosening if I added more spacers? Could I add another threaded nut / old top race to the stack, to lock against the top nut like the current top race is supposed to?