The Cursed PC that *sometimes* boots saga continues, need advice on what to RMA.

[CassWest]

This is a follow up thread to my previous thread. (link) This is a pretty late continuation of the thread(s) but I was on vacation. I will try to keep this thread short and simple to read. 

The issue:
The PC usually does not boot up, pushing the power button does nothing, no fan spin, no LEDs, nothing. If it DOES boot up the PC works perfectly fine, as it should. (this is completely random, does not matter if I use the case power on button or if i manually bridge the +/- on the motherboard to complete the circuit). You have to go through a ritual of turning the PSU off and on again, seeing if it starts and repeating until it eventually powers on (this can take 20+ minutes of attempting)
If the PC boots successfully, I can power it down normally and if I then instantly try to turn it back on it works every single time. If I wait a period of time or if I unplug it from the wall, it does not power back on. 
All parts are brand new.
If I click the flash bios button on the motherboard, I do get the LED to light up.
It handles stress tests (cinebench etc) perfectly. Everything is detected and working as intended when the system is actually on.

What I've tried:
- Updated BIOS to the latest version
- Tested it outside of the PC case by bridging the +/- manually (yes they are the correct pins)
- Turned on/off fast boot
- Removed sticks of RAM and tried different sticks/slots
- Reseated the CPU 
- Reseated everything multiple times
- Visually inspected the pins/cpu
- Visually inspected every connection multiple times, nothing has shorted or is incorrectly plugged in
- Rebuilt the entire system three+ times
- When it powers up, it does so normally, no warning LEDs are showing anything wrong.
- Watched hours of guides to make sure I'm not being an idiot
and more, I've been at this for dozens of hours at this point.

What I need:
What do you guys reckon is the issue? I need to fill out an RMA form on my sisters behalf, but I don't have access to a different PSU or MOBO that is readily available, I'm assuming one of those is the culprit.
If anyone has a lightbulb moment and realizes I'm an idiot, the offer for a $25 LTT gift card still stands. My love for the PC building hobby is diminishing rapidly.

edit: It's 1am, I will read replies when I wake up

Thank you for reading and replying. 

Exact specs of everything:
Lian Li LANCOOL 215
Gigabyte GeForce RTX 3060 WINDFORCE OC 12GB
Samsung 1TB 980
Corsair 750W RM750x (2021)
MSI PRO Z790-P WIFI DDR4, ATX
Noctua NH-U12S
Kingston 16GB FURY DDR4
Intel Core i5-12400F
Windows 11

Why these specs?
She gets a budget of $1k from her job, these were on pretty big sale and came out to less than that.

[orbitalbuzzsaw]

This reads like a motherboard issue to me at first glance. If possible, check with the same parts on another board. 

[cloneman]

make sure the ram is at default speeds (no XMP overclocking etc.) , proper voltage, and disconnect the power and reset switch in case these switches are defective. use a screwdriver or a known good power switch to turn the computer on.

[CassWest]

CONCLUSION:

The motherboard was broken all along, horrible thing to diagnose considering it worked flawlessly when it started. Ended up bringing it to the boutique that sold the parts for a diagnosis, happy I did, ended up getting a new motherboard that was also a slight upgrade. Everything was covered by the warranty.
Thank you to everyone that helped.

Updating this thread and marking this as the "solution" in case anyone googles this and has the same issue, no real help but hey ho. Sometimes shits just DOA and you have to deal with it