Bizarre compatibility issue: PRIME B550M-A WIFI II, Ryzen 7 5700X

[OVERKILL]

My son has an Alienware socket AM4 system w/Ryzen 7 5700X in it that he wanted to migrate to a new Corsair case, so he bought a case, but hadn't realized that the Alienware board had a proprietary back panel, so that took us board shopping. Unfortunately, the case he bought limited him to MicroATX.

Now, the Alienware water cooling setup uses an Intel bolt pattern (as does their board) so he had to order another kit, got a Thermalright setup for it that supports AM4. 

Ordered him the PRIME B550M-A WIFI II, put it all together, no POST. 

Tried a minimal config, still nothing. 

Tried:

-4x different video cards (and two we know worked in the Alienware board)

-2x different sets of RAM, with a single stick tried in every slot

-Re-seated the CPU

-Cleared the CMOS, pulled the battery

Took it all back apart, tested it all in the Alienware board, it worked. 

OK, so, I contacted ASUS through the Chat, they recommended I RMA the board. While I was waiting, I figured I'd order an advanced replacement, which arrived today. 

EXACT same symptoms. 

I cracked open a new PSU (ASUS ROG 850W) and tried that "just in case", still nothing. 

The board powers on, video card fan spins, but USB never initializes. Some quick googling led me to similar symptoms with ancient BIOS versions and incompatible CPU's, but this CPU has been supported since 0303 and both these boards shipped with 3607. 

I've now ordered another CPU to test with, a Ryzen 5 5600XT, just to confirm the board works (which I assume it will), but I have a very hard time with the idea I received two defective boards that exhibit the exact same symptoms. 

Anybody else experience anything like this? 

[OVERKILL]

Update:

The Ryzen 5 5600XT arrived, put it in the board, board POST's no problem. I update the BIOS, put the Ryzen 7 5700X back in, nothing. Put the 5700X back in the Alienware board, fires up no problem. 

So, with this new information, I contacted ASUS Support again and they want me to send the motherboard + CPU to them. Will be interesting to see what they find. 

[Jiaszzz_ASUS]

Hello @OVERKILL 

Thank you for reaching out to us.
Based on your description and follow-up updates, it appears that both tested PRIME B550M-A WIFI IIs are able to boot with the Ryzen 5 5600XT but not with the Ryzen 7 5700X.
Could you kindly provide details on the tested hardware configurations, including the CPU, RAM, PSU, and other relevant components, including the installation slots, BIOS version, and the system’s behavior upon booting? For example: Ryzen 7 5700X / 3080Ti (PCIEX16) / CMK16GX4M2K4266C19 (DIMM A2 & B2) / ASUS ROG 850W / BIOS 3607 / The CPU fan spins after powering on, but there is no display output.

I have also DM you, kindly check it at your convenience. Thank you.

[OVERKILL]

Sure:

- Ryzen 7 5700X / GTX1650DE (PCIEX16) / 9995743-038.A00G (DIMM A2 & B2) / ASUS ROG 850W / BIOS 3607 / The CPU fan and video card fans both spin, USB doesn't get power, there is no display

- Ryzen 5 5600XT / GTX1650DE (PCIEX16) / 9995743-038.A00G (DIMM A2 & B2) / ASUS ROG 850W / BIOS 3607 / The system POSTS normally

- Ryzen 5 5600XT / GTX1650DE (PCIEX16) / 9995743-038.A00G (DIMM A2 & B2) / ASUS ROG 850W / BIOS 3611 / The system POSTS normally

- Ryzen 7 5700X / GTX1650DE (PCIEX16) / 9995743-038.A00G (DIMM A2 & B2) / ASUS ROG 850W / BIOS 3611 / The CPU fan and video card fans both spin, USB doesn't get power, there is no display

Did you want me to list all the video cards and different RAM configs  I tested? as well? Ultimately, everything attached to the board works with the 5 5600XT, but no matter what is attached to the board, not with the 7 5700X. 

[Jiaszzz_ASUS]

Hello @OVERKILL 

Yes, based on your description, it seems that the system fails to pass POST whenever the Ryzen 7 5700X is installed, regardless of the other components. Could you please confirm further details?
1) the specifications of the GPU and RAM that you have tested
If there are any specific scenarios, such as being able to enter the BIOS when installing a 3080Ti (PCIEX16) but failing to do so after a reboot, please do let us know.
Additionally, during the tests you mentioned, did you ensure that no SSD or HDD was installed? Did the issue occur purely while attempting to boot into BIOS?

2) whether the Ryzen 7 5700X was initially installed in the Alienware AM4 motherboard and was able to boot correctly? After encountering the issue, does the 5700X still function properly when tested in the previous board?

3) provide a complete video recording of the startup issue encountered when installing the Ryzen 7 5700X? The video should include details such as the hardware installation location, the steps for clearing the CMOS, the status of the motherboard hardware after pressing the power button, and the circumstances when the system fails to pass POST.

Thank you.