55gr AAC 5.56 split casings after firing

Soooo how big of a problem is this? I'm completely new to firearms so my knowledge is limited. But anyways, I fired a round and something felt off. Looked at the casing and found a crack in the side. Started picking up other casing that had been fired and found two more with cracks in the sides. I've gone through two boxes of this ammo and have yet to sift through the rest of the casings to see if any more have the same cracking in them.

Also, I have an order of AAC 75 grain BTHP match on the way. Should I be concerned with that ammo for any reason?