Binarly has an analysis of the private key leaks that took place at Lenovo in September 2022 and MSI in April 2023.

The report's main finding was that multiple companies were apparently using the same Intel Boot Guard private keys to sign different types of firmware images.

https://binarly.io/posts/Dissecting_Intels_Explanation_of_Key_Usage_in_Integrated_Firmware_Images_IFWI/index.html