There are a few brands of hats where the harness in particularly the chin strap have a tendency to work loose over time.
As a simple tip to stop this happening once you have adjusted the strap correctly use a bit of electrical tape or in my case a small amount of vetwrap to secure the strap in place. You then put the padded strap cover back on over the top and you wouldn’t even know the tape was there.
Glad to see it’s an HS1 issue and not just my hat! Good tip – I’ve just put a plaiting band around it but think vetwrap will be better.
It isn’t just HS1s affected. Most with the metal fastener seem to do the same over time.