Great question — answering as the UphillTech team:
My guess is that someone has stood on the bike while it was locked, which has slightly bend the bolt or mis-aligned the bosses that hold the bolt/pin. Since things are now misaligned, there is binding which prevents the hinge from moving through 180 degrees. Picture #3 shows a threaded hole for a grub screw, BUT I can also see the red of the lock through the hole. That seems wrong, like something is missing. You'll need to inspect closer, and I'd remove that grub screw then try to pull the pin and see if its subtly bent. It could also just be dry-crusty lubricant jamming the works. Good luck !
Full answer by Criggie: https://bicycles.stackexchange.com/a/95394 (CC BY-SA 4.0).
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