So, I gave up wasting time on Googling and restored the instance from backup.
To anyone who reads this - the single most important add-on you must install is the Google Drive Backup. HA is pretty good, but continues to accommodate gotchas like this which can bork the entire system. These seem like edge cases but it is exactly this type of weakness that keeps HA as a niche technology for the tech-aware minority, rather than a product ready for the mass market. /gripe