When subdomain redirection is off, the login form is served inline on
the protected host and submission happens via a same-origin fetch() call
in _script.html.twig. The CSP default-src 'none' was blocking that
fetch (connect-src falls back to default-src).
Add connect-src 'self' to the CSP only when the request is not on the
auth subdomain (i.e. when the inline script is present). On the auth
subdomain the form POSTs normally with no inline script, so the stricter
policy still applies there.
This is the least-privilege relaxation: only same-origin connections,
only on pages that need them.
lyra