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In and of itself, the 10-12 years is not a factor. A lot of ammo is still functioning fine after 60+ years. Especially milspec, some of that is over a hundy.

Having said that, incorrect storage...

Hard to tell from a photo and not being able to move the case around to play the light over it...

I would suggest that this is one of those unusual failures. It would be nice to find out why but in actual reality it probably won't change anything with the rest of the brass even if you did know. ...