fails running RAM 3Ghz XMP, Taichi Ult + 9900K

@bobau The 2 kits having different memory chips on them could well be the problem
in your case. Unfortunately this happens often with RAM manufacturers and is why
tech sites and gurus always recommend matched kits or at least consecutive serial
numbers. When the RAM manufacturer runs out of a given RAM die they will replace
their stock with whatever fits the data sheet for the best price and not bother
changing the RAM model and in most cases don't even bother to add a revision number.

I have even run into "matched" kits that have samsung on one module and hynix on
the other, clearly one was made before the switch and one after. This was back
in the days of DDR 400 however and I have not seen the issue pop up since (I
was not alone and there were complaints).

IMO they should at the very least add a "rev 1.0" then "rev 1.1" etc every time
they change RAM dies out maybe even as far as "rev 2.0" if the dies come from
another vendor entirely.

I am having the absolute same issue - running the absolute same system Z390 Taichi Ultimate, with 64GB Ram Trident, and i9900K.

Unfortunately switching off those 2 settings did not help. Have you done anything else? Perhaps updating BIOS, I still have not updated it, somewhat reluctant to do it.

I saw on reddit that you replaced the board at some point, did those changes fix the original slot 3 issue or only fix the timing issue on the replacement board?

One more suggest: if you are using Windows OS always install INF Drivers for the motherboard. It extremely increases the speed of the system.

Latest INF driver from the oficial site:
- INF driver ver:10.1.17968.8131_Public (2019/5/13)

https://www.asrock.com/MB/Intel/Z390%20Taichi%20Ultimate/index.asp#Download

Sorry to necro, but I am now having this exact same issue on a Z390 Taichi (non-ultimate). I picked up an additional 2x16GB of DDR4-3000, booted up, and saw all 4 sticks had dropped down to 2133mhz. Tested same as OP and found that and any stick I put into DDR4_B1 causes the motherboard to panic at boot a few times, eventually getting into bios which shows the ram has underclocked itself down to 2133mhz (same slot as OP). Did anyone ever find a solution to this other than board replacement? Frustrating as I am out of the newegg RMA period at this point and will likely have to wait for ASRock to respond to assist, hopefully it is a known issue at this point.

Cheers