Case separations like that are usually caused by excess headspace. The firing pin impact drives the case forward, primer fires, is pushed back against the bolt face, case expands against the chamber walls, case head is pushed back, reseating and flattening the primer and stretching the case wall exactly where yours failed.
Not caused by a hot load.
If you were to section the heads of some fired cases you might find the internal ring of incipient separation. You might also find it with a paper clip probe as suggested above.
You cannot trust this batch of cases. They my not be trustworthy for use in your rifle.
Get new ones.
As described above, for sizing purposes, treat the cases as if they were rimless, and size to suit your rifle. Belted cases are known for short case life unless reloaded as if rimless.