I have a relatively new install. When I upgraded to 6.10 my server only shows a single core, but when I rolled back to 6.9 it properly showed the 4 core 8 thread CPU. Mind you this is a I7-6700K. Unraid even see's the correct CPU in the dashboard but only shows the single core, CPU 0.
The motherboard is a MSI Z170A GAMING M7 (MS-7976) with a BIOS of BIOS dated: Thu 28 Jun 2018 12:00:00 AM CDT.
I have kept the server current on Unraid supported releases, so I am now at 6.11.5, but haven't had my quad core CPU shown since 6.9. I have attached diagnostics to review. I can see when it boots that there are errors on the CPU, but I'm unsure how to resolve those.
hawkeye-diagnostics-20230413-0829.zip