Relax, Apple disabling a GPU core in A12X only to enable it in A12Z is quite common
A report published by Notebookcheck citing TechInsights revealed that the SoC dies of the A12Z, used in the 2020 iPad Pro just released, and the A12X, in the two year old outgoing model, are identical. This has caused some confusion on how this could be, and naturally, immediate hot takes on Apple holding back only to up-sell a feature later. In this case, this isn't true, and some number of Apple enthusiasts are going to stumble into a new phrase: 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. Another mass consumer example that gamers of a certain generation may remember well is the PS3's Cell processor. At the time, the combined silicon budget of the CPU and GPU was quite hefty, and this led to quite a defect rate, which drives down yield, how many working chips you can harvest. By disabling one SPE core on the Cell, Sony was able to ship more viable processors. This is also a very common practice among GPU vendors. Oftentimes, a lower end GPU is the same physical chip design as the higher end part, but one or more defective core units, whatever the manufacturers branding for it, was disabled. By disabling a GPU core on A12X, Apple was able to increase the number of producible SoCs early in the 7nm era. Now that we're two years in, 7nm yields are far better than they were, and they were able to enable the core with better yields, and gave it a minor name revision to note that the performance did get marginally better. That's it, that's all. But the hot takes will still flow.