No surprise here. GM has the worst after treatment. They have their dicks in it so deep, they need to know every little thing everything's doing. They also have the most harsh drive cycles. There could be 4 other things wrong right now too but it hasn't tested for them yet.
I've sent a dozen or so trucks out with the preface of something will surface after this repair. So you can either pay me to drive it for a few hundreds miles or you can take it back