I have been loading 9mm Luger almost 40 years a never had a case head separation while resizing the case!
It was not a big problem but I only had 2 more cases to load.
I measured the ID and it was a little smaller than 5/16 so I put the die in the lathe and used a 5/16 tap and with the tap still in there I used a small punch through the hole where the decapping rod goes and with little effort pushed the tap and broken case out of the die.