PC randomly loses power and restarts despite a 1200W PSU and UPS

My three-year-old PC has suddenly gone black and restarted twice within two weeks. The first failure happened while playing a demanding modded Minecraft server, but the second occurred while I was only browsing YouTube. Event Viewer only reports an unexpected shutdown after each incident, with no warning immediately beforehand. The system has an Intel i9-13900K, RTX 4090 OC, 64 GB of DDR5-6400 memory, a Gigabyte Z790 motherboard, and a Thermaltake GF3 1200W 80 Plus Gold power supply. It is also connected to a CyberPower 1000W UPS. The CPU has a mild undervolt, and I'm trying to determine whether this is a hardware, stability, or power-related problem.