Interesting. I've never tried steel case in my CPX2.
If it's an issue, give the company a call. Mine broke a firing pin during a dry fire - just shot the end of it across the room. Called them up, was on hold for less than 2 minutes, gave them my SN, to which they replied that there had been some internal upgrades in the 4 months since my pistol had been manufactured so they would just send me an entire new upper assembly and a shipping label to send the old one back with. 2 days later it was in my hand and works very well.