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One power outage later, I've learned that #update-grub isn't enough when booting in #EFI mode and changing the partition that holds the grub menu configuration. One also needs to run #grub-install to update the EFI grub.cfg to point to the first grub.cfg

https://wiki.debian.org/GrubEFIReinstall

And of course, the power outage had to happen just as I had left for Germany and Spiel. Good thing the rest of the family wasn't home either...