I thought I had this, but I didn't; the SATA SSD was irrelevant. What's relevant is that the 306 BIOS broke access to my NVMe boot drive. Exact symptoms:
* The BIOS sees an NVMe drive as connected.
* It doesn't see anything bootable on it.
* Going into an EFI shell, any attempt to access data on the drive, even just "dump a raw block from this device", fails with I/O errors.
* This is true whether or not any other drives are connected.
Reverting to the 302 BIOS fixed this, but doing so was a huge pain. To do that, you have to use the WinFlash.exe utility with the /nodate parameter, which means that, if your boot drive doesn't work, you have to take all drives out of the machine, put in a spare drive, install Windows to that, then run the utility with the older BIOS, which you have to have just kept lying around just in case. The BIOS flash utility won't work, because it refuses downgrades, which is inconvenient when the most recent BIOS is broken.
I was disappointed, though, because support told me that it was impossible to downgrade the BIOS (which it isn't) and claimed this was clearly a hardware failure, and since the machine is out of warranty, they proposed I spend money to send it in for them to give me a quote for repair, even though nothing was actually wrong with the machine. A more paranoid person might suspect that putting up a BIOS which breaks these machines around the time the warranty on nearly all of them would be expired, and then leaving it broken, would be a great way to make money on servicing the machines...