Well, the title sums it up. It's a cheap (~50 EUR) KTM helmet. This is reproducible, is happens on every ride: if I adjust it to fit well at the beginning of the ride, it's loose after a while. If I tighten it during the ride then it is way too tight at the start of the next ride.
Two One hypotheses:
- The straps are getting wet with sweat and loosening, then constricting again when they dry. Nylon does in fact absorb water and stretch when it gets wet, so this is the more likely cause.
2. Exertion causes vasodilation. If you've had the sensation of your necktie getting tighter under stress, this is the cause. Your body is pumping more blood to your head and the veins are expanding.
Seriously, hasn't every cyclist experienced this? Every single helmet I have ever worn in the last 30 years does this. The nylon straps stretch when wet and shrink when dry. I keep hoping some engineer will fix this and replace the chinstrap material with something that doesn't stretch. Kevlar straps would be a lot more expensive but wouldn't have this problem. Alternately, surrounding the strap with a wicking material to keep it from getting so wet would also help.
Bottom line - there is nothing wrong with your helmet that isn't wrong with every bicycle helmet ever made. I adjust by tightening the straps mid ride and then loosening them before the next ride because they tighten up so much while drying out.
The retightening is the surprising part. I propose a test. Adjust the helmet, then ride without using it for long enough that it good notice. Wear a spare ideally, but rollers or conditions where you'd be happy without a helmet are other options. Try the helmet for size when you get back. If the fit has changed with the helmet at home, it's your head. Otherwise it's the helmet.
A helmet that fits quite precisely could reseat while riding,and not necessarily in predictable ways given sweat and vibration.
Another test would be to ride gently on a cool day to avoid sweating - does the adjustment fail then? That suggests vibration (on one of mine there's vertical adjustment in a plastic band at the back of my head; if I put it on with that too high and it drops, the helmet loosens.
Different helmets have different strap arrangements even if the material is the same, so you may find you're just unlucky with yours.