I came across this thread and wanted to share my experience.
A few months ago I bought a used Bickerton 1607, my 2017. A couple of days after I got it I noticed a big play in the main hinge and realized that the previous owner had installed non-original bolts. I took them out and ordered the original service kit... After two months of usage, the bottom bolt broke off.
I wrote Tern and Bickerton and no one really answered my question about why they tend to break. I also asked about the steel grade of service bolts, but no one could answer. I was only offered another service kit.
My initial thought was that the pin is employed to protect the aluminium frame and hinges of a folding bicycle, in the same way that fuses are used to protect electrical wiring. It gets broken first to protect the rest from tearing.
But after I realized that Tern/Bickerton have upgraded the hinge FBL 2 Gen 1 to Gen 2, it was clear to me that they knew gen 1 had been a faulty construction and the bolt was made of bad quality steel. Sadly, there's no way I could modify the hinge to Gen 2 so I had to find another solution.

Since I didn't want to spend more money on this cock-up bolt again and I didn't want to risk any serious injury while riding my bike, I decided to do it my way. I ended up finding a turner who modified a truck's engine 10.9 grade steel bolts and created exactly the same pins as the originals. So far, they have been working fine, they sit tight. But if this happens again and one of them gets cut off, my last idea is to drill out the hinge hole, enlarge the diameter from m6 to m7 and use a thicker bolt that would be significantly better durability. (Hopefully, the hinge frame will be strong enough since drilling the hole will make the hinge walls thinner)