Thanks for the support.
I have contacted both Tern and Evans Cycles (where I bought the bike). I shall let you know how I get on.

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)

10.9 might be a little over killed; I used them as upgrade from 8.8 on my race car (12.9 was a no go as while stronger, it was brittle).

As you mentioned, the issue is frame preservation, if you put a strong pin, the loads will lead to aluminum hinge cracking and fitting a bigger pin would compromise the frame due to wall thickness thinning. If I were you, I would convert it as a serviceable item; make sure it is easy to swap and like a chain, swap the pin for a new one as part of a servicing schedule. You can most likely get a bag of 316 dowels for little money.

I thought the same thing but..... I'm not so sure.
Over time Bickerton changed the hinge name from Gen 1 to Gen 2 and aside from changing the inner hinge structure, the appearance/shape remains the same.
While for Gen 1 you need two partially threaded pins (the top and the bottom), in Gen 2 only one solid pin goes across the entire hinge.
They claim it doesn't seem to be failing like Gen 1 did.
So yes, I would think the pins indeed protect the frame in Gen 1, but if Bickerton purposely changed these two bolts into one, maintaining the hinge and frame shape, I'm pretty sure it's not about protection but the method of transferring loads in the hinge.
And what's crucial here - the cylindrical part of the pin is the most loaded part while the pin breaks where the threaded part meets the cylindrical section..So that's a design flaw to my taste.

(for some reason I can't upload any attachment/jpeg. tried many times, it stucks on 90%, anyone knows why?)

File size too large? I've had this problem.

(bold above) Best quality bolts, like engine head bolts, have the shoulder at the minor diameter of the thread; On these, the nut will slide over the shoulder. This results in the bolt strain being distributed over the bolt length. Most bolts have a shoulder at major diameter of the thread, and these always-always-always have a stress concentration at the transition of shoulder to threads; Most of the stretch is in the threads, versus the more rigid large shoulder.

Used to be, good quality socket wrench extensions were the same, the shaft diameter same as distance across male flats. Most extensions now have a shaft same diameter as distance across male points, easier to fabricate, and more rigid in torsion, but they are more prone to breaking at the transition to the male flats.

From the description you gave, it is clear that Gen2 is better constrained. on Gen1, you get 2 pins that may be more forgiving in term of alignment tolerances and ease of assembly but Gen2 is better structural design.
As Duragrouch mentioned the 2 pieces construct with threaded pins without undercut is a recipe for disaster with stress raiser.

I've been trying to upload different images showing the hinge cross section and the differences between gen 1 and gen 2 but for some reason I can't do it. Tried even to reduce the jpg size to 40kB, still stucks at 90% and it's not going further....

Hi, that would be really interesting! In the family, we have 4 Tern Verges P9/P20 and two of them have a broken bolt on the bottom of the hinge. I was actually wondering if it is possible to change the frame from OCL1 to OCL2, and wonder what is the exact difference. Maybe you could use another service to upload the photos like imgur?

All of our Terns have the extra weld at the bottom of the hinge (see post #18 in this thread), so I am confident that the frames will not snap.

Take a look at the technical drawings of both frames where the fbl2 gen 1 and gen 2 are shown.
Gen 2 has one pin that goes across the hinge, gen 1 has two pins from both sides (upper and lower).

Edit: You can't use gen 2 pin on gen 1 frame, it won't fit. I verified and asked at Bickerton's too. What You can do is to find a turner and let him do the exact looking pins of 10.9 or 12.9 grade steel. I did that and since then my bike has been working fine.