I have 2 helmets; both the same brand (Bell). I am not sure of the model, but I know it's a mtb helmet that I rip the visor off of. It has lots of vents.
Anyway, after I've gone on a few rides, the chin strap mysteriously starts getting looser on both of them, to the point where it has to be tightened. In fact, this has happened to me with every helmet I have had. I do readjust the ratchet somewhat frequently, as i sometimes wear a buff or skull cap and my head seems to swell with the heat.
I have never seen anyone else have this problem.