Case head separation cause by week recoil spring? I’d like to hear an explanation of that.
Nickel plated brass is harder. True! Probably why it’s separating on the third loading. Nickel isn’t as pliable as brass so it would be more likely to “crack” after being expanded then squeezed back down. Also Glocks have an unsupported chamber adding to the stress on the case head. Glocks also have a “generous” chamber allowing brass “nickel” to expand more than other pistols.
Guns are not dangerous! People are! RKBA!