I do not know, but, what I do know is: Had you stopped shooting at the first sign of a problem you could have called Winchester with the problem and a lot #. But, still, there is a chance, your cases are once fired, once fired cases behave in a predictable manner as in a new/unfired/minimum length case is as good as it gets then comes once fired. You have 18 once fired cases, if I had 18 once fired cases that were suspect I would use my 338/06 or 35 Whelen versatile full length sizer die to neck-up the suspect cases, if after necking the 8mm cases up to 338/06 or 35 Whelen (neck dimensions) I found splits in the necks, I would conclude the cases were brittle.
When forming: I have formed 280 Remington to 35 Whelen, 30/06, 30 Gibbs and 338/06 without splits, EXCEPT for nickel cases, the price of forming shiny cases is close to 40% failure.
Then there is the crush test.