discussed this with chx in #drupal after fatal errors were triggered by #715108: Make Merge queries more consistent and robust → --> #839006: SPL autoloader should use require_once to be more robust. → .
we should add a note to say that moving a class requires an update to rebuild the registry, chx suggested here would be a good place:
Not all content is available!
It's likely this issue predates Contrib.social: some issue and comment data are missing.