I finished the migration (that VM got rather mangled and I ended up having to restore it from backup)
Once the upgrades were completed, I still had another problem with an apparently out-of-sync resource which I reported in this thread, and then was able to reproduce in this github issue.
In short: the out-of-sync counter itself gets out of sync, and can report a value greater than zero even when the entire verify pass succeeds with no mismatched blocks detected.