I think one challenge this thread has is the use of the word "popped". It leads some to think the recessed nut cracked/broke. Did it? Post 8 states "The bolt was still inserted into the fork crown and examination shows threads still look fine." To me this reads as the nut not being damaged. So what is it? Broken nut or just a nut that loosened and unthreaded completely? Unthreading is far more likely. But this process is not a quick one. Certainly not the process that "pop" would best describe.

BTW the image of the caliper mounting/center bolt suggests some thread form flattening. This is often from a threaded fitting being stressed while loose. The nut "floats" on the bolt causing the threads to become somewhat rounded. Andy