You could be correct about the undersized power supply, as AMD recommends an 850W for the 6900XT.
https://www.amd.com/en/products/graphics/desktops/radeon/6000-series/amd-radeon-rx-6900-xt.html
However, a stock reference 6900XT is rated at 300W total power, and the 12600K at 150W, and as these are the two biggest power hungry components in the system and fall well below 650W total, your power supply is a Gold rated unit from a reputable manufacturer that could handle that load.
Is the GPU a reference or an overclocked model? Have you attempted to replicate the issue running a system stress test or benchmark? Taken any measurements or captures using software of the total power wattage used by the CPU/GPU during these high load conditions? How about temperatures? What refresh rate is the monitor, and have you tried setting a FPS cap?
Ryzen R7 5700X | B550 Gaming X | 2x16GB G.Skill 3600 | Radeon RX 7900XT
Ryzen R7 5700G | B550 Gaming X | 2x8GB G.Skill 4000 | Radeon Vega 8 IGP
Ryzen R5 5600 | B550 Gaming Edge | 4x8GB G.Skill 3600 | Radeon RX 6800XT