Found it! The answer to my home server’s weird error messages during OS updates. Turns out, the Ubuntu default is to look around for other operating systems you may have installed, so it can put them all on the OS selector boot time menu. It makes sense, if you have a dual-boot laptop, but not for a server which only has a single OS. Apparently my LXD containers all look like external disks to the OS prober. Sounds like a minor bug to me. Probably will be fixed in 6 months.