With the latest AMD Chipset drivers installed from the AMD website and running Minecraft, try switching to the High Performance power plan before you launch the game and let me know if that helps.
When running games, the cores on the second CCD should be mostly parked. However, if the CPU utilisation becomes high enough, it is normal and expected for one or more cores on the secondary CCD to wake up and become utilised. Based on what you both described, it sounds like this is what is happening. It is normal, so please don't worry.
To verify at least some of the cores are parked, you can launch Resource Monitor in Windows, switch to the CPU tab and under Views select small. Now scroll down to core 8-16, launch your game and once the game is at the main menu, alt tab out to resource monitor and you should briefly see the word Parked against cores 8-16.
Please note that once you are back at Windows desktop, no cores should be parked. This is normal and expected.