I have exactly the same issue. For my iMac (Retina 5K, 27-inch, 2017) 3,5 GHz Intel Core i5 quadricore / On Big Sur : 11.2 My iMac crash especially during the night when I'm not using it. Each day I come back to my office, I notice my iMac has reboot. I hope to see soon a solution by Apple.