Although admittedly a dull knife, I was sharp enough to grasp that I need to carry both passports in future, but thanks for reminding me

Perhaps my wordiness obscured the intent of my post: what was the logic of stamping the new passport?

I was hoping to discover from y'all some new reasoning beyond the possibilities I had already deduced: A) the visa should not have been moved to the new passport because the IT system has irrevocably married the visa to the (old) passport number; or, B) the IT system allows the visa to be re-assigned to the new passport number but the Immig officer neglected to make the change; or, C) the Immig officer made the change but the IT system is/was buggy.

Thanks again for the replies.