Z390 Taichi won't post if memory in DDR4-A slots

I recently purchased a Z390 Taichi, Intel Core i5-9600K, and 2x16GB DDR4-3200 Ballistix Sport memory. That combination worked fine on the Taichi. I ended up exchanging that that board for another z390 Taichi due to a malfunctioning onboard HDMI. Unfortunately the second Taichi would not post if there was any memory in the DDR4-A1 or DDR4-A2 slots. Everything worked just fine with memory in the DDR4-B1 and/or DDR4-B2 slots but anytime I populated one of the DDR4-A slots the motherboard would power up and cycle through a bunch of post codes, eventually pausing on "4C" before recycling. It never reached the BIOS screen.

Thinking something was physically wrong with this second motherboard, I exchanged it for a third Z390 Taichi. Much to my surprise, I had the exact same issue with the third board. I upgraded the BIOS to 4.10 to see if that would help, but unfortunately it did not.

At this point I am stumped. Anybody have any thoughts on where I might start debugging this? Any suggestions are welcome. I am not looking forward to a fourth 100-mile round trip to the dealer to try to figure out what is wrong.

What happens when the memory is installed in A2 and B2 ? That is where the manual states to put it.