It looks like AMD's latest AGESA ComboAM5PI 220.127.116.11 firmware has done more harm than good. As tipped by hardware leaker chi11eddog (opens in new tab), the firmware reportedly disables some cores on the Ryzen 5 7600X, prompting various motherboard vendors to pull the update from their websites.
The flawed firmware doesn't affect all Ryzen 5 7600X, one of the best CPUs. It only impacts the samples that leverage a dual-CCD design. Given the Ryzen 5 7600X's hexa-core, 12-thread configuration, a single CCD is more than sufficient. However, some Ryzen 5 7600X processors arrive with two CCDs under the hood. Obviously, AMD fused off the additional CCD, so you can't just magically unlock more cores.
Chipmakers such as AMD, Intel, or Nvidia aim to maximize silicon production and habitually recycle dies that don't meet the requirements for specific models into their lower-tier products. That's how dual CCDs found their way into the Ryzen 5 7600X. In retrospect, AMD utilized the same strategy for previous Ryzen 7 5800X and Ryzen 5 5600X processors.
It would seem that the AGESA ComboAM5PI 18.104.22.168 firmware has difficulties distinguishing the single-CCD Ryzen 5 7600X samples from the dual-CCD ones. It's plausible that it doesn't correctly recognize the CCD and, as a result, disables one of the processor's cores. According to the chi11eddog, the firmware locks down Core0.
MSI and ASRock have removed the firmware from their X670 and B650 motherboard product pages. On the other hand, Gigabyte still has the firmware up for download on some of the brand's 600-series motherboard support pages, but you should avoid it at all costs.
If you recently upgraded your motherboard's firmware and have noticed an evident performance loss on your Ryzen 5 7600X, you should roll back to the previous firmware. A fix is reportedly on the way, and the new firmware should arrive in the next couple of weeks.