This does seem related to the tile size. The crash happens with 32x32 tiles. I ran several more tests with varying tile sizes, and was able to render without crashes using CPU+GPU, although there is no measurable increase in render speed using larger tiles over solo-3090 GPU renders:
GPU only, 32 x 32 tiles: 24 seconds
GPU+CPU, 64 x 64 tiles: 23 seconds
GPU+CPU, 256 x 256 tiles: 1 minute 11 seconds
GPU+CPU, 32 x 32 tiles: instant power-off crash during initial tile renders
I’ll try 16x16 next.