Hey, wasn't able to find any mention of this, but here's the problem.
There's a motherboard ROG STRIX 470-F GAMING for AMD processors.https://rog.asus.com/motherboards/rog-strix/rog-strix-x470-f-gaming-model/helpdesk_cpu
The motherboard has a an on-board AMD RAID controller, which offers level 0, 1, 10. It works, at least with Ryzen 7 1700 processors.
After a planned upgrade to Ryzen 7 5800x, which is listed as supported on the URL above contingent on BIOS updated past or equal version 5809, it works in general, except the AMD RAID. The BIOS POST screen just doesn't show up. I ensured that the RAID is selected as the SATA mode in BIOS. I also went ahead and applied a few earlier BIOS versions than the last one, yet new enough to be higher than the one that is listed as supporting this CPU first. Same result.
But it takes just to remove the 5800x and put the 1700 back in and the AMD RAID POST menu magically appears on the next boot after setting the SATA to RAID. And on the opposite side, just remove the 1700 and put the 5800x and no RAID for ya.
I reviewed the FAQs and documentation on top of searching the forums, although I haven't read every word, at the very least I've read everything with keywords like RAID, CPU, compatibility and so on. There is a weird statement in the RAID manual one can download from one of the links from the URL above that says, I quote, "Due to CPU behavior, CPU RAID functions with Intel® CPU RSTe only supports Intel®Core™ X-series Processors (6-core or above) and Intel® SSD modules."
Although it's not related to my situation at all, it hints that as a phenomenon CPU and RAID BIOS incompatibility can be observed.
Overall, I feel disappointed but at least I've ordered an external PCIe RAID card and will have to rebuild the array & restore from backups so I can use the CPU I bought.
Everyone else in a similar situation -- beware.