Monterey install goes back to Big Sur, looks successful

I let the Monterey upgrade run (from the Software Update control panel). It runs the whole course, gives no errors or warnings... and then boots back into Big Sur (it is not a boot drive confusion).

Here's the log. I had zero issues upgrading from Catalina to Big Sur on this system (10850k, RX 580, Asus Prime Z490-P, 64GB RAM). Any clues?

My kexts: LILU, VirtualSMC, AppleALC, EnergyDriver (Intel Power gadget), Realtek RTL8111, SMCProcessor, SMCSuperIO, CPUFriend, custom USBMap, CPUFriendDataProvider, Whatevergreen, DroboTBT (had issues with Drobo until I put the kext right into EFI), TrustedDataSCSIProvider (same), HibernationFixup, NVMeFix, all fully updated

EDIT: tried once more, same thing