Damn, didn't think to look into this of course but now that I have the Gigabyte X570s Aero G, I am having the exact same issue.
3rd slot from the CPU is doing it's own thing and isn't detected within ICUE. Funny thing is, within ICUE, it suggests that it's the 2nd DIMM slot from the CPU that isn't being detected but it looks like it's actually the 3rd slot from the CPU.
I'll keep my fingers crossed for a timely resolution from Gigabyte.