Asus Hero Z890 Slot B2 Problem

[Wojti85]

Motherboard: Asus ROG Maximus Z890 Hero (BIOS: 1101)
Memory: G.SKILL DDR5 48GB (2x24GB) 8000MHz CL40 Trident Z5 RGB XMP (F5-8000J4048F24GX2-TZ5RK)
Processor: Intel Core Ultra 285K
PSU: Corsair AX 1600i - 1600W
Graphic Card: ROG Strix GeForce RTX™ 4070 Ti 12GB GDDR6X OC Edition


I have a problem with XMP-8000 Mhz G.Skill RAM (2x24GB - F5-8000J4048F24GX2-TZ5RK). My BIOS is 1101 (latest version). I tested both modules (RAM-1 and RAM-2) individually in program PassMark MemTest 86 11.1 in order to obtain "PASS" in all four passes of the test. Due to errors at slot B2 with XMP profile, I ordered a second set of the same G.Skill memory kit (RAM-3 and RAM-4), which also showed errors in slot B2 with XMP profile. I did't change anything in XMP profile, except lowering Mhz. The results of testing RAM in memtest86 are following:
Slot A2 (XMP with factory settings - 1.35V)
RAM-1 - XMP-8000 Mhz - PASS
RAM-2 - XMP-8000 Mhz - PASS
RAM-3 - XMP-8000 Mhz - PASS
RAM-4 - XMP-8000 Mhz - PASS
Slot B2 (XMP with factory settings - 1.35V)
RAM-1 - XMP-8000 Mhz - FAIL, XMP-7333 Mhz - PASS, 5700 Mhz - PASS
RAM-2 - XMP-8000 Mhz - FAIL, XMP-7433 Mhz - PASS, 5700 Mhz - PASS
RAM-3 - XMP-8000 Mhz - FAIL, XMP-7566 Mhz - PASS, 5700 Mhz - PASS
RAM-4 - XMP-8000 Mhz - FAIL, XMP-7566 Mhz - PASS, 5700 Mhz - PASS
Slot B2 (XMP profile with increased VDD-1.45V and VDDQ-1.45V)
RAM-1 - *XMP-8000 (1.45v) - FAIL, *XMP-7600 (1.45v) - PASS
RAM-2 - *XMP-8000 (1.45v) - FAIL, *XMP-7800 (1.45v) - PASS
RAM-3 - *XMP-8000 (1.45v) - FAIL
RAM-4 - *XMP-8000 (1.45v) - FAIL
Slot B1 and B2 - 4000 Mhz - PASS (RAM-1, RAM-2, RAM-3, RAM-4)

I ordered a second motherboard (the same model and BIOS version: 1101) and made few tests, which also showed errors on slot B2:
*Slot A2 (XMP with factory settings - 1.35V)
RAM-3 - XMP-8000 Mhz - PASS
RAM-4 - XMP-8000 Mhz - PASS
*Slot B2 (XMP with factory settings - 1.35V)
RAM-3 - XMP-8000 Mhz - FAIL, XMP-7566 Mhz - PASS, 5700 Mhz - PASS
RAM-4 - XMP-8000 Mhz - FAIL, XMP-7566 Mhz - PASS, 5700 Mhz - PASS

I chose RAM-3 and RAM-4 as they showed better results on slot B2. Both motherbords with RAM-3 and RAM-4 failed test with XMP-8000 Mhz in slot B2. Both RAM-3 and RAM-4 passed test at XMP-7566 Mhz in slot B2, which is exactly the same result in both motherboards. Also both RAM fail test at any higher value above XMP-7566 Mhz. Hovewer, increasing VDD and VDDQ to 1.45V, helped to get a stable 7866Mhz/7766Mhz on RAM-3 and RAM-4, but still there are error beyond this speed (failed to reach PASS on 8000 Mhz).


What is the problem here: motherboard, memory or CPU?

PS
Both G.Skill memory kits were standard and despite being on QVL, they fail to reach stable 8000 Mhz. Shoud I use CUDIMM memory kit with Client Clock Driver (CKD) to improve stability at default XMP (without increasing Voltages or doing some other OC settings, which are not required according to Standrad XMP profile)?

[David (PassMark)]

8000MT/s is current bleeding edge. Instability isn't really a surprise.
And I think some of the RAM vendors have over promised and under delivered. Maybe their marketing departments got carried away.

Intel, to their credit, don't promise the Ultra 285K can run at anything above 6400MT/s. So you are well beyond the official specs.