I've done the RAM dance with every combo of A0/A1/B0/B1. BMC is 12.61.35, and tried both BIOS F19 and F20
Are you sure it's not doing POST just fine, but it's the Display Output / KVM that is broken ?
I don't have this Board and I don't have IPMI on my ASROCK Rack B450D4U-V1LQ5 but for sure the AMD Ryzen 7 2700X was an absolute PITA with recent BIOS Versions on that ASROCK Motherboard.
No VGA Output, no Rear USB Ports working, PCIe x16 Slot defaulting to Bifuration x4x4x4x4.
Only Solution in my Case was to install a x8 Riser Cable in the x8 Slot and wire an old external GPU there. Then I would finally get a Video Output and I could see that it was posting properly. Just Boot was NOT configured to boot from any Device so my LiveUSB couldn't be found on the DHCP Server Static Leases

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Not sure if relevant to your Case, but in my case I also thought it wasn't posting, but the DEBUG LED (with the BIOS Codes) would do its "Looping" and the LED Bios Code turn off after a while, indicating normal Boot.
Board was Posting, it was "just" the VGA not outputting any signal with this BIOS Version & CPU Architecture

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EDIT 1:
@lklaw, did you try F18 ?
it could be that you have an Issue with the latest AGESA 1.2.0.E
I have found at least one Thread online where at least one User had Issues and downgrading to AGESA 1.2.0.C seemed to help.
Since F19 has been upgraded to AGESA 1.2.0.E, maybe it's worth checking F18 to see if at least that one works for you.