On sso.wikimedia.beta.wmflabs.org login page, the "Powered by MediaWiki" icon does not render:
Compared to the normal login page:
The <img> element has src="/en.wikipedia.beta.wmflabs.org/w/resources/assets/poweredby_mediawiki.svg" (note the single slash – this is a relative URL with a wiki prefix, not a protocol-relative one). It should either be rewritten to use https://en.wikipedia.beta.wmflabs.org/, or the SSO domain should serve that URL. I'm not sure if this is a MediaWiki bug, CentralAuth bug, or a configuration bug.