Nice job finding a work-around. I ended up returning my board last week and replacing it with a different model (a Prime Z590-P) that worked out of the box, but good to know that a work-around exists and glad my experience helped you find it. Bad enough that the auto-detection logic is busted...
Yeah... contemplating that myself. I don't want to re-rebuild yet again, but may be the only option. I'd love if there were a firmware solution forthcoming, but I know better than to hold my breath waiting for it. :p
You can do a quick test just by taking the M.2 drive out and trying to boot...
I've been having this exact same issue, and went through similar steps to diagnose, including getting a replacement motherboard which also has the issue. I found a work around, but you're not going to like it (I know I don't): this only happens if I have an M.2 SSD installed on the board, but I...