Revision log message

Some new information came to light on this issue, will followup here soon.

Finally found a solution to this, in a hook_node_presave for example a client did this to fix it:

    $entity_log_message = $entity->get('revision_log')->getString();
   // Use a database query to get the current revision ID of a node.
    $connection = Database::getConnection();
      $query = $connection->select('node_field_data', 'nfd')
      ->fields('nfd', ['vid'])
      ->condition('nfd.nid', $entity_id)
      ->condition('nfd.langcode', $langcode);

    $current_revision_id = $query->execute()->fetchField();
    if (isset($current_revision_id) && is_numeric($current_revision_id)) {
      $current_revision = \Drupal::entityTypeManager()->getStorage('node')->loadRevision($current_revision_id);
      $current_revision_log_message = $current_revision->get('revision_log')->getString();

      if ($current_revision->isPublished() && $moderation_state == 'draft') {
        if ($entity_log_message == NULL && $current_revision_log_message != NULL) {
          $entity->set('revision_log', ' ');
        }
      }
    }

This likely will lead to a better fix.