I hope someone can help me, I've bought my first Rpi a few months ago and I haven't been able to use it at all because of this issue.
Specs: Raspberry Pi 4 - 4GB
OS: Raspberry Pi OS with desktop Fully updated with
sudo apt full-upgrade
Nothing else has been installed
This is the case and fans I'm using:

The pi reboots constantly, sometimes it was as much as once every hour or less, now it takes around 4 hours I think.
After a lot of testing I realized the problem occurs when I select a DMT resolution (any resolution) even tho I have no HDMI connected. I do this so I can connect through VNC and can interact with the desktop.
I took the pi to the service of the place where I bought it before realizing this was the issue and they did not found any problem, when I contacted them again with my new find they said they couldn't do anything because in their initial inspection they couln't find any error.
I tried changing to the FakeKMS Driver following this answer https://raspberrypi.stackexchange.com/a/88184 but it only made the pi take a bit longer to reboot.
I looked at all the usual logs people look at to diagnose crashes (syslog, dmesg) and couldn't find anything meaningful. If it helps I can get some logs and upload them anyway.
Is this normal? Or could this be hardware related?