See PHI2190, etc. When Phacility hosts on the m4.large chassis restart by surprise, they don't (or, at least, don't reliably) run bin/host upgrade --hold-versions and remount volumes.
The trigger system switched from "upstart" to "sysetmd" in rCORE75c788bd and I probably tested it at the time, but evidently missed something.
I'm going to kick this host (secure-01a4c8f6) a bit and see how far I get in figuring out what's going on.