New thing.
I have seen this happen a number of times over the years in both new white steel mainsprings and the old blue steel springs. Regarding the blue springs I feel it had more to do with slight surface surface corrision than anything else. I have fitted new, old stock blue steel springs US, English and Swiss made which looked good with no kinks or rust and have self destructed within a few days of fitting just like the photo's show.
At times it has been at the first winding !
It was certainly not from hand winding the spring into the barrel as a winder was always used.
I have also seen the modern Swiss mainsprings do exactly the same thing, there was a replacement spring for a '60's Omega calibre that did the same thing.
I never found any reason for the breakdown, but it seems it must be metal fatigue caused in the rolling process, who knows maybe it is the first or last piece off the roll and is stressed.
I have also seen a few of the same exploded springs in a clock
David