Do I need to flip a BIOS setting for ECC RAM?

Yes. On the Asus Pro WS X570-Ace, ECC is not enabled by default. You have to go into the BIOS and turn it on by navigating to Advanced → AMD CBS → UMC Common Options → ECC Configuration and setting ECC Mode to Enabled. This board only supports unbuffered ECC (UDIMMs), so double-check your RAM kit before buying.