I think I have discovered the cause here... I had the owners application uninstalled, which means that the PhabricatorRepositoryCommitOwnersWorker tasks weren't being queued... and I'm guessing that as a result the PhabricatorRepositoryCommitHeraldWorker tasks weren't being queued either?

It looks like there are a couple of logical issues which can cause us to complete a PhabricatorRepositoryCommitOwnersWorker task without marking it as completed and without queueing the followup. Notably this block:

if ($repository->getDetail('herald-disabled')) {
  return;
}

If we exit early there, we'll never make it here:

$commit->writeImportStatusFlag(
  PhabricatorRepositoryCommit::IMPORTED_OWNERS);

if ($this->shouldQueueFollowupTasks()) {
  PhabricatorWorker::scheduleTask(
    'PhabricatorRepositoryCommitHeraldWorker',
    array(
      'commitID' => $commit->getID(),
    ));
}

Is it possible that's the explanation, rather than "Owners" being uninstalled? I don't immediately see a way that Owners not being installed could cause this, but maybe I'm just missing it.

(Those snippets are from PhabricatorRepositoryCommitOwnersWorker.)

Yes, you are correct. This was likely caused by the if ($repository->getDetail('herald-disabled')) { return; } logic.

same issue here