Steel cases split. It happens. I've had WWII steel cased rifle ammunition do this and I've had brand new Russian 7.62, 5.45 and .223 do it as well. I've reloaded steel cased .223 where I got splits on the initial factory load, splits on a few at the second reload ,third reload and so on. Some cases never split, even after 5 or 6 reloads...
Don't sweat it.
DD