That's the interesting thing. The log file was owned by www-data. And the job queue was running as www-data. But somehow, the log file was flagged as being non-writeable. Even though other events were being written to the file. Very bizarre chain of events, anyway.
Thanks again for the suggestions - reminding me of the very existence of the job-queue daemon was the key piece of information that let me figure out what had happened, and fix it. And I did add that note to the installation instructions suggesting that the status of that process be checked after starting, to avoid any such issues for anyone in the future. No idea how it happened, so I can't prevent that, but it can be mitigated.