That's interesting. Is the c-ring plastic or metal?
It's metal!
My last thought is that when preloading at home, things are not
really preloaded, and when riding, the spacers compress more, and that leads to the play?
I've tried again, compressing the top cap by having my gf push on the stem while I screw it in, and after 5 mins riding, there's play again, so I'm really puzzled... It's the 3rd chiner that I built, and I've never had these issues.
I think that I'll take it to my LBS, unless someone sees something I'm missing