X299 Taichi RAM detection issue

Hello,

I have an ASRock X299 Taichi motherboard with an Intel i9-7920X CPU.

Only two RAM slots (A1 and A2) are being detected and initialized.

The other six slots (B1/B2, C1/C2, D1/D2) seem to work physically, but the system will not boot if RAM is installed in them.

Whenever I put memory in those slots, the system shows Dr. Debug code 53 (memory initialization error).

When I install all 8 RAM sticks, the DRAM Tweaker in BIOS can still detect and list every module correctly.

In CPU-Z and HWiNFO, all 8 sticks also appear, and the total shows as 96 GB installed.

However, on the BIOS main screen, only the two sticks in A1/A2 are actually recognized, and the other slots simply show "None"

In Windows, only 24 GB is detected and displayed in Task Manager.

So even though 96 GB of memory is physically installed, the system only reports 24 GB available. (A1 and A2 only)

What I Have Tried:

1) Flashed BIOS to oldest and latest versions, and some versions in between as well.

2) Replaced CMOS battery with a new one and performed CMOS reset using both button and jumper.

3) Tested with a different PSU and different GPU.

4) Tested with different RAM sticks (Corsair and Crucial).

5) Tried different RAM combinations

6) Tried to boot with only 1 RAM stick, one by one in each slot. Only in A1/A2 it boots, and in others, it gives Error 53

7) Verified all RAM sticks on another H270 motherboard - they work fine.

8) Removed CPU, cleaned contact pads, and reinstalled it properly.

9) Intel Processor Diagnostic Tool (IPDT) passes all tests.

10) Tried different CPU cooler pressures - from very tight to gently placed.

In short: The board can clearly detect all DIMMs through SPD (as shown in CPU-Z, HWiNFO, and BIOS DRAM Tweaker), but it only initializes the A1/A2 slots.

Maybe slots are not dead, they are simply not being recognized properly somewhere.

Windows Task manager - https://snipboard.io/PnTfym.jpg
CPU-Z               - https://snipboard.io/an6Bv3.jpg
CPU-Z all 8 slots    - https://snipboard.io/hVk245.jpg
BIOS "Main"          - https://snipboard.io/uGtN3B.jpg
BIOS "Dram Tweaker" - https://snipboard.io/QX5CEe.jpg
Error code 53        - https://snipboard.io/uV5TLa.jpg

Yes, I have already removed, cleaned, and reseated the CPU multiple times, and also experimented with different cooler mounting pressures - from very tight to very light contact.
Unfortunately, the result has been exactly the same each time: only A1 and A2 slots are detected and initialized, while B1/B2, C1/C2, and D1/D2 remain completely undetected.

If it were a pressure-related contact issue, I would expect at least some change in behavior (for example, B1/B2 occasionally appearing), but no matter what I do, only the A channel ever initializes successfully.

This makes me think the slots or memory channels themselves are fine physically, but something is preventing the other channels from initializing at all.