Problem solved: when I commissioned some nodes, I decided to ‘remove’ the second NIC from the MAAS list of interfaces. That really screwed up the underlying hardware - the underlying Supermicro BMC reported it only had 1 physical NIC. For some reason, 16.04 (Xenial) did not complain, but when I booted 18.04 (Bionic), it got into a reboot loop each time it loaded the initial ramdisk (boot-initrd).
Seems like there is an upstream problem with Bionic, though it seems like an edge case.