New PC randomly loses video under load (RTX 5070?) but PC stays on

I built this PC about a week ago, and since yesterday I've been having a strange issue.
While playing CS2 (and later FiveM), my monitor suddenly goes black, but the PC stays powered on. The keyboard LEDs stay on Caps Lock still lights up, but according to my friends watching my Discord stream, my mouse stops responding. the GPU fans keep spinning (sometimes even faster), and I can still hear Discord for at least a moment. The monitor itself works fine because I tested it with my PS5 using the same HDMI cable.
Here's what I've tried:
Checked all HDMI and power cables.

Updated to the latest NVIDIA drivers.

Entered BIOS (the PC booted normally afterward).

Ran FurMark (GPU stress test).

Ran a CPU stress test at the same time.

The temperatures did not appear to be overheating.
However, after the stress tests finished (or while the system was under heavy load), the problem happened again: the monitor lost signal, but the PC remained powered on. My friends were watching through Discord screen share and noticed the stream started lagging right before it happened.
Other information:
Motherboard: MSI B650 Tomahawk WiFi

GPU: RTX 5070

PSU: Corsair RM850e 850W (80+ Gold)

Connection to monitor: HDMI

The monitor works perfectly with my PS5.

I also noticed a warning icon on "AMD Graphics" in Device Manager.

At this point I'm not sure if this is:
a GPU hardware issue,

a driver problem,

an AMD iGPU/NVIDIA conflict,

motherboard issue,

or something else.

Has anyone experienced something similar or knows what I should test next before replacing hardware?
Any help would be greatly appreciated.