I just noticed that my Matomo instance had the IPv6Usage plugin running.

The plugin doesn't officially support Matomo 3, hasn't been adopted by anyone and removed from plugins.matomo.org

    "require": {
        "piwik": ">=2.12.0"
    }
}

But as the plugin has an open-ended require it hasn't been deactivated by Matomo.

I think Matomo should at least show a warning to superusers as subtle bugs may be caused by them and maybe this way more people would try to adopt popular plugins that aren't maintained anymore.