Start off with this 9to5Mac post from Chance Miller, “Report claims new iPad Pro’s A12Z Bionic chip is just a ‘renamed A12X with an enabled GPU core’”:

In its press release for the new iPad Pro, Apple said that one of the changes with the A12Z Bionic processor was the addition of an eighth GPU core. Notebook Check, however, claims that it has confirmed the A12X Bionic processor from 2018 actually features 8 GPU cores, but that one is disabled. This would imply that Apple has simply enabled that eighth GPU core and changed the marketing name of the processor.

While this may be true, it’s worth reading this thread from Quinn Nelson:

I see a lot of people getting irrationally frustrated that the A12Z is just an A12X with an extra GPU core unlocked. This might seem weird those who don't know how manufacturing ICs works, but Apple is doing what literally every silicon manufacturer does. Lemme explain:

— Quinn Nelson (@SnazzyQ) March 27, 2020

In a nutshell, Quinn explains that this practice is longstanding and widespread in the industry and, likely, nothing new for Apple. And, more importantly, nothing sinister.

Another take on the so-called practice of “chip binning”:

Chip binning is a common practice in the silicon industry, and the theory goes like this: For repeatable structures like a GPU core, each added core adds to a potential defect rate. By disabling one core by design, you can ship more viable dies at a given target performance.