In November 2021, I upgraded my main desktop computer with an AMD Ryzen 5950X CPU, an Asus Prime X570 Pro motherboard, and a Corsair CMK32GX4M2D3600 2 x 16GB DDR4 RAM kit. I installed the two sticks in the DIMM_B2 and DIMM_A2 slots, per the motherboard manual. Everything functioned just fine with DOCP settings with the memory running at its rated 3600 MHz speed.
In December 2022, the same memory kit went on sale for about 35% off, and I decided to buy another to expand the RAM on this computer to 64GB. I installed the two sticks in the remaining DIMM_B1 and DIMM_A1 slots on the motherboard. The computer started crashing with a Windows BSOD after benign tasks such as Zoom video conferencing. I ran CPU-Z which showed that the old 2021 kit was made by Micron and was single-rank, but the new 2022 kit was made by Nanya and was dual-rank. All 4 DIMMs had identical SPD timings for XMP 3600, and identical voltage of 1.35V.
I then ran memtest86 from a USB stick. I had to force it to run in parallel on all CPUs, as by default it would only run on a single CPU on this motherboard for some reason. I left it running a few hours. I was horrified to find the following result :
The test had more than 10006 errors in under 3 hours, and aborted. Something was very wrong with this picture.
I pulled out all 4 sticks of RAM, and installed just the new dual-rank kit in DIMM_B2 and DIMM_A2 slots on the motherboard. I ran memtest86 again in parallel. After about 5 hours, it passed with flying colors.
I then repeated the test with just the old single-rank kit in DIMM_B2 and DIMM_A2 slots. Again, memtest86 passed with flying colors after 5 hours.
Finally, I had the idea to install the new dual-rank kit in DIMM_B2 and DIMM_A2 slots, and the old single-rank kit in DIMM_B1 and DIMM_A1 slots. I ran memtest86 overnight. After about 11 hours, it also passed in full.
1) Corsair changed manufacturer for its RAM kit, and went from single-rank to dual-rank, without changing the part number.
2) When mixing single rank and dual rank memory, the Asus Prime X570 Pro motherboard needs the dual-rank kit to go in the B2/A2 DIMM slots, and the single rank kit to go into B1/A2 DIMM slots. This is the only configuration that works reliably. The reverse does not.