We get to learn that the Apple A12X Bionic has eight physical GPU cores, but one of those cores is disabled. The disabled core is enabled in the new A12Z Bionic that powers the 2020 iPad Pro. While there could be several reasons for this decision, the most convincing explanation is that it would save Apple from having to develop a new A13X chip while a more powerful A14X is already in the pipeline for a late 2020 release.
https://www.notebookcheck.net/The-Apple-A12Z-Bionic-SoC-is-just-a-renamed-A12X-with-an-enabled-GPU-core.458883.0.html