Неверный заголовок ответа устанавливается контроллером embed

[Falco]

Это исправлено сейчас:

Mitigate XSS Attacks with Content Security Policy

We just landed support for CSP frame-ancestors directive. It’s disabled by default for now behind the content security policy frame ancestors site setting. You can add domains to the list using via /admin/customize/embedding as always. This directive will be enabled by default in the next release cycle.