Just a bit of weirdness. I park 5 miles from work and ride the rest of the way using different routes to break up the tedium. I rode the bike to work on Monday. I took a photo of the bike Monday morning just as I was headed out for it's maiden ride and the bolt is intact. As I returned to my van Monday night, the bike got quite wobbly as I dismounted. I figured the hinge bolt just broke at that moment. Before I loaded the bike in the van, I saw the one hinge bolt was snapped off. I walked back about 30 feet looking for the bolt head but didn't see it. Who knows where it broke or landed. Well, today, 3 days later, I'm unloading another bike to ride to work and I found the bolt head sitting on the rear bumper of the van.

After more thought, I'm of the opinion this live hinge pin and 2 bolts are not a great design.All the vertical weight is supported by the two 5 (or 6) mm bolts in a sheering and twisting action. If the shaft extended through the 2 arms on the rear triangle, the larger shaft would support the weight instead of just the smallish
bolts. Or a larger single bolt (same diameter as the hinge shaft) through the rear triangle arms and through the frame would be far stronger and wouldn't weigh much more and would be much easier to service. The counter sunk bolts do look nice but a protruding bolt head and nylock nut would not interfere with other components. (well, maybe if you use two chain rings, not likely) I read where someone as asking about using a Brompton for loaded touring, I can see a major issue if you broke one of the stock bolts on the road somewhere.

I have no idea of how often riders break the hinge bolts. My experience has been 1 15 mile ride and it broke. The bike is quite used. I don't know it's history.

Alternative hinge bolt? "Long Shoulder bolt". with a hex socket end or a normal hex end.
The challenge would be finding one with suitable dimensions.